Speech rate

Speech rate was found to be a strong predictor of word-internal /t,d/ deletion in the complete dataset, with more deletion occurring in faster speech. The effect of rate reflected the fact that the mean three-word speech rate for deleted tokens was higher (6.87 syllables/sec) than the mean rate for nondeleted tokens (5.93 syllables/sec). In fact, the mean rate for deleted tokens was higher than the mean rate for any other variant, as shown in Table 3. As with the overall mean, for each speaker the mean absolute three-word speech rate for deleted tokens was higher than the rate for tokens of other variant types.

Mean absolute and relative three-word speech rate for tokens of each variant (sec)

There was a consistent effect of speech rate, regardless of the rate measure used; however, more local contexts produced larger contributions to predicting deletion, with the largest contribution coming from the three-word rate measure. The three-word measure was used in all subsequent analyses, and results using other measures will not be reported.

Relative speech rate also predicted deletion. As shown in Table 3, the mean speech rate relative to the speaker's mean rate was higher for deleted tokens (.707) than for nondeleted tokens (−.178). In fact, the relative speech rate was a predictor of deletion for tokens in onsets as well as for those in codas when these subsets were tested separately. For tokens in syllable onset position the mean was 1.39 for deleted tokens and −.036 for nondeleted tokens, so that for onset tokens there was a greater difference between the relative rate means than between the absolute rate means. For deleted tokens in codas, the mean relative speech rate is −.160 and for nondeleted tokens it is −.718.

Dysfluencies

The results show that tokens in words preceded by a silence had a slightly higher deletion rate (19.3%) than tokens in words not preceded by silence (17.0%). However, there was no effect of a preceding silence after controlling other factors. On the other hand, the absence of a following silence predicted deletion in the dataset, reflecting the fact that words not followed by a silence had a deletion rate almost twice as high (18.6%) as words followed by a silence (9.7%). A following silence also inhibited deletion for tokens in onsets (p < .001; β = −.838) and codas (p < .001; β = −.633), when these subsets were tested separately.

No other dysfluencies that we examined individually had an effect on deletion, although tokens in words adjacent to dysfluent productions were almost always associated with lower deletion rates than tokens in more fluent environments. Lexical repetitions had lower deletion rates in both the first and second occurrences of the word than words that were not repeated. Words containing /t,d/ tokens that were preceded or followed by filled pauses also had lower deletion rates compared to words not adjacent to fillers. A following lexical cutoff inhibited deletion in words with /t,d/ tokens relative to /t,d/ words not followed by cutoffs as well, but when the word containing a /t,d/ was preceded by a cutoff, deletion was promoted relative to tokens in words not preceded by cutoffs.

Age and gender

Age predicted deletion in the dataset, although the contribution to modeling deletion was not great and the effect was not seen in all subsets of tokens, suggesting that other factors interact with, or may account for, the observed effect. The effect of age reflects the fact that younger speakers had higher rates of deletion (19.0%) than older speakers (15.4%), consistent with the findings of Guy (1992) for word-final /t,d/ deletion. Two factors that might provide an explanation for age differences are the absolute and relative rate measures discussed earlier, because both measures affected deletion and may be different in the two age groups. The effect of age was not due to absolute speech rate differences in the two age groups, because older talkers spoke, on average, faster than younger speakers. However, younger talkers' mean speech rate for deleted tokens exceeded their overall mean speech rate by only .59 syllables/sec, whereas for the older speakers the difference was .87 syllables/sec. That is, the younger speakers tended to delete at rates that were on average 30% lower, relative to their mean rate, than the older speakers. The lower threshold for deletion for younger speakers combined with their greater range of rates largely accounts for the higher deletion rate among these speakers.

There was an additional interaction between age and the syllable position of the alveolar stop tokens. For tokens in syllable codas the older speakers had higher rates of deletion (30.9%) than younger speakers (24.3%), and older speakers produced these tokens at faster rates than younger speakers did, consistent with the older speakers' overall higher speech rate. However, after controlling rate, age did not emerge as a significant factor influencing deletion for tokens in syllables codas. On the other hand, for tokens in syllable onsets, age predicted deletion (p < .001; β = −.608), even after controlling speech rate. Although the older speakers had lower rates of deletion than the younger speakers, these tokens were, once again, on average produced at faster rates by older speakers than by younger speakers.

There was no effect of gender on deletion rate. However, like Zue and Laferriere (1979), who reported higher internal /t,d/ deletion for read speech in the environment

in female speakers than in male speakers, the deletion rate in this flapping environment in our data was also higher for females (24.4%) than for males (20.2%).

The results of analyses of extra-linguistic variables on deletion are summarized in (2).

Word class

Word class predicted deletion in the complete dataset (p < .001, β = −.566), reflecting the fact that there was more deletion in function words (29.2%) than in content words (12.8%). Note that word class is highly correlated with word length, word frequency, and predictability from neighboring words, as shown by the differing means and standard deviations of function and content words shown in Table 4. For all lexical variables, there was a broader distribution of values in content words than in function words. Because of the correlation of word class with other lexical factors, further analyses with lexical variables were performed separately on tokens in function words and tokens in content words, as well as on the complete dataset. Correlations between word class and other structural factors will be considered in the analysis of phonological variables.

Mean values (and standard deviations) of some lexical variables by word class

Word structure

There was substantially more deletion of tokens in monosyllabic words (22.2%) than in polysyllabic words (14.7%) in the overall dataset. This binary length variable proved significant in predicting deletion in the dataset (p = .009, β = −1.25), as well as for tokens in content words (p = .001, β = 1.570) and function words (p = .001, β = 1.570) when these subsets were tested separately. Interestingly, the binary length effect was in the opposite direction for content and function words. As seen in Table 5, there was more deletion in monosyllabic function words and polysyllabic content words than the reverse. This result appears to be a consequence of structural differences between function and content words. Function words are fairly evenly distributed between monosyllabic (43.5%) and polysyllabic words (56.5%). However, in polysyllabic function words, 99% of all stops occurred in a syllable onset, whereas in monosyllabic function words, all stops occurred in a coda. This confound largely explains the higher deletion rate in monosyllabic as opposed to polysyllabic function words, because (to anticipate the discussion of phonological variables) there was a much higher deletion rate for coda tokens than for onset tokens.

Deletion rates by word length and word class

Table 5 shows deletion rate for function words as a function of two properties, length in syllables and syllable position of the alveolar stop. When the subset of function word onset tokens is tested, there is a small effect of length in syllables on deletion (p = .031, β = .312), reflecting the fact that onset tokens in longer words are more likely to delete than those in shorter words. The effect comes from some three- and four-syllable function words—the indefinite pronouns (e.g., somebody and anybody)—that have unusually high rates of deletion of /d/ in onsets, although deletion of /t/ in onsets of longer words was not unusually high. The high deletion rate in the indefinite pronouns reflects idiosyncratic reduced pronunciations of these words (e.g., somebody,

; nobody,

; anybody,

). After taking the identity of the class of indefinite pronouns into account, there was no effect of syllable length on onset tokens in function words, and hence on function words in general.

Note from Table 5 that as content word length increased, the deletion rate for tokens in syllable codas also increased. In contrast, tokens in syllable onsets had about the same deletion rate in words of all lengths. In fact, length in syllables was not a significant predictor of deletion before controlling syllable position of the stop token. When syllable position of the word-internal stop was taken into account, syllable length predicted deletion in content words for tokens in codas (p < .001, β = .600), but not for tokens in onsets. For coda tokens, deletion likelihood increased with word length.

The effect of syllable length on tokens in onsets and in codas was reversed when we considered the effect of ordinal syllable position on deletion. The data showed a correlation between content word length and the syllable in which the stop occurred. In longer words, stops tended to occur in later syllables. In content words, the ordinal syllable in which a stop occurred predicted deletion for onset tokens (p = .002, β = .225), but not for coda tokens. The effect reflected the fact that there was more deletion of tokens in syllable onsets when they occurred later in a word.

The results of analyses involving word structure variables are summarized in (3).

Word probabilities

Word probabilities investigated included the word frequency of words containing word-internal /t,d/ tokens, frequencies of adjacent words, and the predictabilities of /t,d/ token words from adjacent words. The frequency of the word containing a word-internal alveolar stop did not predict deletion in the overall dataset, nor did it predict deletion in function or content words when these were tested separately. In the complete dataset, the mean word frequency of words with deleted tokens was higher (3.39) than words with nondeleted tokens (3.13), but this was the result of the higher mean frequency of function words which, as we have seen, had higher deletion rates than content words.

In both function and content words there was little difference between the mean frequency of deleted and nondeleted tokens, as shown in Table 6. If an effect of word frequency is to be found that is independent of word class, it is more likely to be detectable in content words than in function words, because of the greater range of frequencies in content words. Nonetheless, when content words were tested separately they showed no effect of word frequency. However, as Table 6 also shows, mean frequencies of words with and without deletions differed by syllable position (onsets and codas) and preceding phonological context (V_ and C_ environments) of the stop token. Interestingly, when the four subsets of content words defined by syllable position and preceding segment class were tested separately, frequency of the token word did predict deletion, but only for onset tokens in V_ contexts of content words (p < .001, β = .746). This context largely comprises the V_V⁰ flapping environment in words like better and lady, where V⁰ represents an unstressed syllable; but the context also includes words in nonflapping environments, in words like attract. Indeed, word frequency predicted deletion in the V_V⁰ flapping environment in content words (p < .001; β = .798) when this subset was tested separately, and these words appear to be the source of the effect in the broader class.

Mean log frequency of deleted and nondeleted tokens by word class, syllable position, and preceding phonological context

In analyses including frequencies of words adjacent to words containing a /t,d/ token, subsets based on syllable position and preceding context were tested separately, as they were in token word frequency analyses. Analyses of following word frequency showed an effect for content word onset tokens in V_ contexts (p = .01; β = −.268) and in the V_V⁰ flapping environment (p = .008; β = −.303). Note (from the negative beta values) that the effects reflect less deletion of tokens in words followed by higher frequency words than of tokens in words followed by lower frequency words. No effects of preceding word frequency were found in any content or function word subsets tested, or in function words.

The failure to find word frequency effects more broadly, seems at odds with previous findings (Jurafsky et al., 2001). However, note that in the environments in which word frequency effects were found, deletion is associated with higher frequency token words (w), but with lower frequency following words (w + 1). The word frequency results can thus be readily interpreted in terms of word predictabilities, defined in terms of the probabilities of w and w + 1 [cf. equation (1)]. Given the effects just reported, we thus expect an effect of p(w|w + 1) on tokens in V_ environments of content words. Higher predictability from the following word results from two factors: (1) higher word bigram frequency of the pair 〈w, w + 1〉; and (2) lower frequency of w + 1. This is precisely what the results indicate if we additionally assume that higher w frequencies will generally be associated with higher 〈w, w + 1〉 frequencies.

When word predictabilities were substituted for word frequencies in the regression model, the predictability of a word containing a word-internal /t,d/ from the word that followed it in speech p(w|w + 1) emerged as a predictor of deletion in the overall dataset (p < .001, β = .268), reflecting the fact that there is more deletion of tokens in more predictable words than in less predictable words.

The (log) mean predictability from the following word was higher for deleted tokens (−1.95) than for nondeleted tokens (−2.31). When word class subsets were tested separately, p(w|w + 1) also predicted deletion for tokens in content words (p < .001, β = .283), but not in function words, so that the overall effect came from the effect in content words. As expected, the effect in the overall dataset could once again be isolated to tokens in V_ contexts in syllable onsets in content words (p < .001; β = .554). Predictability from the following word in speech also predicted deletion in tokens in the V_V⁰ flapping environment in content words (p < .001; β = .662), reflecting the fact that deletion was more likely in more predictable words than in less predictable words.

There were no effects of predictability from the following word on any other subset defined in terms of preceding context and syllable position of the word-internal /t,d/ token. There were also no effects on deletion of the predictability of a /t,d/ word from the word that preceded it in speech in the overall dataset in any subsets defined using preceding context and syllable position in content words.

The results of the effects of word predictability measures are summarized in (4).

Syllable position

After controlling for lexical and extra-linguistic variables, syllable position of the alveolar stop token was a predictor of deletion for the overall dataset (p < .001, β = −1.174). The effect reflected the fact that there was a higher rate of deletion for word-internal /t,d/ tokens in syllable codas (29.1%) than for tokens in onsets (11.1%). For both tokens in onsets and codas there was more deletion in function words than in content words. However, there was a stronger effect of word class on tokens in onsets than on tokens in codas. For tokens in onsets, the odds of deletion in content words were about half the odds of deletion in function words (e^β = .52); in coda tokens, the odds in content words were about two-thirds the odds in function words (e^β = .67). The stronger effect of word class on onset tokens was a result of the high deletion rates of /d/ in indefinite pronouns (cf. the discussion of word length effects). Taking the identity of indefinite pronouns into account eliminates the interaction between syllable position and word class.

Prominence

Prominence level predicted deletion in the overall dataset (p < .001, β = .662) and for tokens in onsets (p < .001, β = .931). There was only a marginal effect in coda tokens (p = .06). Note that the prominence variable analyzed all monosyllabic function words as nonprominent. Including this variable in the model, the deletion rate for tokens of /t,d/ in nonprominent syllables was 21.2%, and the deletion rate in prominent syllables was 9.5%. Prominence on the syllable containing the stop token also predicted deletion in content words (p < .001, β = .807), but not in function words, when these subsets were tested separately. In all subsets, an effect of prominence reflects the fact that tokens in nonprominent syllables were more likely to delete than tokens in prominent syllables. The failure to find an effect in function words can be explained by the fact that most function words are monosyllabic, and so unaccented by definition, leaving few function words to be distinguished by prominence. When effects were found in function word subsets, they were consistent with the results for corresponding content word subsets.

When the four subsets of content words defined by syllable position and preceding segment class were tested separately, all subsets showed effects of prominence. Deletion was more likely in nonprominent syllables for onset tokens in postvocalic position (p < .001; β = .845), for onset tokens in post-consonantal position (p = .002; β = .637), and for coda tokens occurring after a vowel (p = .004; β = 1.37). However, deletion was less likely for tokens in codas of nonprominent syllables in C_ contexts (p = .002; β = −1.26). The effect also held when the stop occurred intervocalically in content words (p = .005; β = .876), but not in the extended flapping environment V{n r l}_V⁰.

When the syllable preceding the alveolar stop was prominent (e.g., in supposedly, water), the deletion rate was 17.0%, and 14.0% when nonprominent (e.g., in attempt, validate). Prominence level of the syllable preceding the /t,d/ token predicted deletion in the overall dataset, but not for tokens in onsets or codas when they were tested separately. For onset tokens, there was an interaction between prominence on the /t,d/ syllable and prominence on the preceding syllable, reflecting the fact that there were lower deletion rates when the preceding syllable is nonprominent and the /t,d/ syllable is prominent, as shown in Table 7. The interaction was not significant for coda tokens, although the pattern was the same. Thus, as has been proposed for flapping, only the prominence level of the syllable containing the /t,d/ token affects deletion. Finally, the prominence of the syllable following the alveolar stop had no effect on deletion for tokens in codas, in onsets, or overall.

Alveolar stop identity

After controlling for extra-linguistic and lexical variables, there was a marginal effect of alveolar stop identity on deletion (p = .052, β = −.171), with deletion more likely for /d/ tokens than for /t/ tokens. The weakness of the effect is compatible with the fact that the overall deletion rate was about the same in /t/ tokens and /d/ tokens, as seen in Table 8. Alveolar stop identity strongly interacted with word class, and to understand the interaction, function and content word subsets were tested separately.

Deletion rates for /t/ and /d/ by word class and syllable position

As shown in Table 8, tokens of /t/ and /d/ are distributed in about the same proportion in function words (/d/ = 25.3%) and content words (/d/ = 21.8%), although there was more /d/ deletion in function words and slightly more /t/ deletion in content words. The content word effect disappeared, however, when this subset was tested separately. On the other hand, the effect of stop identity for tokens in function words remained when function words were tested separately (p = .003, β = −.629), reflecting the higher deletion rate of /d/ than /t/ in these words. The effect in function words can be further limited to words with tokens in onsets (p < .001, β = −1.89), which had an unusually high deletion rate of 33.3%. This subset of words consisted of only 19 types (430 tokens), which can, except for three tokens, be further grouped into three subclasses: indefinite pronouns ending with -body (e.g., somebody, nobody), contracted forms with 've (e.g., could've, would've), and contracted forms with n't (e.g., wouldn't, shouldn't). The deletion rate in the subclass of indefinite pronouns was 30.2% and the deletion rate for the contractions was 42.0%.

As discussed in the section on word length effects, high rates of deletion in the indefinite pronouns accounted for the effect of word length in functions words, and it was suggested that the high deletion rates were a consequence of idiosyncratic pronunciations of these words. We would like to test whether effects of these subclasses, found there and again in this analysis, were a result of the phonological properties of the words or their lexical identities. If we compare deletion rates of these subclasses to phonologically similar words, the deletion rate of the function word subclasses is much higher than that of comparable words. For example, the deletion rate of /d/ in the V_Lax_iy⁰ environment (e.g., idiot, comedy, which are comparable to the -body words) was only 8.7%. [The mean frequency of V_Lax_iy⁰ context words (3.15) was comparable to the mean frequency of -body words (3.14).] The deletion rate of /d/ in the

environment, which is comparable to the contractions, was 8.6%. These deletion rates are very similar to the deletion rate of /t/ in the V_V⁰ environment in function words (6.6%). The greater likelihood of deletion of /d/ than /t/ in onset tokens in function words is thus most likely attributable to idiosyncratic pronunciations in these word subclasses and is not the result of phonological context or of a difference in deletion between /t/ and /d/.

Recall Rhodes' (1992) claim that the greater ease of flapping of /d/ over /t/ leads to greater rates of deletion of /d/ over /t/. To test this hypothesis, an analysis was performed on tokens in flapping environments. No effect of stop identity on deletion for tokens in the intervocalic flapping environment (i.e., V._V⁰), was found. However, there was more deletion in VdV⁰ tokens (21.3%) than in VtV⁰ tokens (12.3%), as Rhodes observed for flapping. There was also no effect of stop identity on deletion for tokens in the extended flapping environment V{l r n}_V⁰ when this set was tested separately. In the extended environment, there was more deletion of /t/ (23.9%) than of /d/ (19.3%).

Differences in the patterning of /t/ and /d/ were also proposed in Zue and Laferriere (1979), who suggested that /t/ is more likely to delete than /d/ in a nasal release context V′_N (where N represents a syllabic /n/). Although there was a higher deletion rate for /d/ (34.3%) than for /t/ (14.3%), generalizations are problematic, because the number of types in the dataset with this context was small (n = 17; although there were 294 tokens). The high deletion rate for /d/ can once again be attributed to deletion in some specific words, in this case, the -n't contractions (e.g., wouldn't). If we exclude these function words, then the deletion rate for /d/ drops to 4%.

Consonant/Vowel: Preceding context

Proportions of deletions for tokens preceded by consonants and vowels in the overall dataset, and in the onset and coda subsets, are shown in Table 9. Interestingly, in the complete dataset, a preceding vowel promoted deletion relative to a preceding consonant (p < .001, β = −.763). Note that this overall pattern reflects the pattern in onsets, because 72% of tokens in the dataset (and consequently 52% of all deletions) were in onsets. In postvocalic position, deletion was promoted for onset tokens in all words (p = .003, β = −.346), as well as for onset tokens in content words (p < .001; β = −.832). A preceding consonant was shown to strongly promote deletion when the stop was in a coda for all words (p < .001, β = 1.97), but when it was in an onset, deletion only proved significant in content words (p < .001; β = 2.20).

Deletion rates by segmental context

Consonant/Vowel: Following context

Unlike results from studies of word-final /t,d/ deletion, the class of the following segment as consonant or vowel did not predict deletion in the overall dataset after controlling for syllable position. We assume that this is because, word-internally there is a strong correlation between syllable position and following segment class; recall that in our dataset, codas cannot be followed by vowels, by definition. Note that the class of the following segment also did not predict deletion for tokens in onsets when this subset was tested separately.

Preceding vowel contexts

The analysis of /t,d/ deletion in tokens following a vowel compared contributions from three variables: (1) the binary classification of vowels as lax and nonlax; (2) the classification of vowels following Zue and Laferriere (1979), based on the presence or absence of a high off-glide; and (3) the phone bigram frequency of the alveolar stop and the preceding vowel.

For onset tokens occurring after a vowel, in the complete dataset there was no effect on deletion of the two vowel classes tested or on preceding phone bigram frequency. Interestingly, when postvocalic onset tokens were tested separately, an effect of preceding phone bigram frequency emerged (p = .004, β = .615), but the two vowel classifications of the preceding vowel remained nonsignificant. The effect of preceding phone bigram frequency reflected the fact that the mean bigram frequency of deleted onset tokens was higher (4.64) than for nondeleted onset tokens (4.57) in postvocalic environments. Preceding phone bigram frequency was also a predictor of deletion for prevocalic onset tokens in content words (p = .005, β = 1.813) and in function words (p = .005, β = 1.813). For coda tokens, neither preceding phone bigram frequency nor either of the verb classifications predicted deletion in the overall dataset or in function or content words.

Consistent with Zue and Laferriere's (1979) findings, we found less deletion in the intervocalic flapping environment for tokens preceded by a vowel with a high off-glide (10.0%; e.g., liter, lady), than for those with no high off-glide (16.9%; e.g., matter, ladder). However, there was no general effect of this vowel classification on deletion of tokens in the flapping context. In this environment there was also no effect on deletion of the distinction between lax and nonlax vowels or of preceding phone bigram frequency.

Following vowel contexts

Contrary to the findings for prevocalic position, vowel bigram frequency was not a significant predictor of deletion in onsets for tokens in prevocalic environments. The mean following vowel bigram frequency (i.e., the mean bigram frequency of the /t,d/ and the following vowel) was about the same for deleted tokens (4.86) and nondeleted tokens (4.81). Deletion rates for tokens in onsets followed by a lax vowel were lower (10.9%) than deletion rates for nonlax vowels (13.4%), but there was no contribution to predicting deletion from this vowel class for tokens in onsets followed by a vowel. There was also no effect for prevocalic onset tokens of the classification of vowels by off-glide. Following vowel bigram frequency and classes of following vowels are not applicable to coda tokens, which cannot be followed by vowels.

Preceding consonant contexts

For /t,d/ onset tokens preceded by a consonant, there was a robust pattern of deletion when the preceding consonant was a homorganic sonorant (/l/, /n/, or /r/), as shown in Table 10. Examples of deletion in this environment from the corpus include children, ended, wanted, and started. As indicated in Table 10, chi-square tests revealed no significant difference between the rates for the /n/_ and /r/_ environments, nor between those for /r/_ and /l/_, although /n/_ differs from /l/_ (p = .03). Environments with these homorganic sonorants produce more deletion than the /s/_ environment (p = .009) and all other preceding segments. The difference between word-internal /t,d/ segments preceded by homorganic sonorants and other consonants was a predictor of deletion for /t,d/ tokens in onsets (p < .001; β = −1.39). After controlling for preceding consonant context using this binary variable, there was no additional contribution to predicting deletion in onset tokens from preceding consonant bigram frequency.

Deletion by preceding consonant for tokens in onsets (with vowel context included for comparison)

Rates of deletion of onset stops preceded by a homorganic sonorant (16.4%) were similar to flapping rates in these contexts (19.1%). However, although deletion was highest in /n/_ environments, flapping was highest in /l/_ and /r/_ environments. It is interesting to note that the higher rate of deletion after a homorganic sonorant was similar to that after a vowel (11.8%), also a flapping context. In fact, the difference between /l/_ and V_ contexts was nonsignificant by chi-square test, but rates in /n/_ differed from rates in V_ (p = .03).

The findings differ from previous studies of final /t,d/ deletion in which obstruents (stops and fricatives) were found to promote deletion, although sonorant consonants (nasals and liquids) inhibited it (Guy 1980, 1997; Labov et al., 1968; Neu, 1980; Wolfram, 1969). Our finding that a homorganic sonorant promotes deletion is consistent with the findings of Fasold (1972), Neu (1980), and Zue and Laferriere (1979), who each found that a preceding homorganic consonant promoted deletion both for final and nonfinal /t,d/ (in Zue & Laferriere, only /n/ was tested). However, our results revealed that homorganicity alone is not an effective predictor, because deletion rates for tokens in onsets after a homorganic obstruent were low. Rather, homorganicity is relevant when the consonant is also a sonorant. The results are summarized in (5). The differences between the findings of this and earlier studies relate to the fact that syllable position of the alveolar stop was not previously taken into account; as we show next, the conditioning factors differ for tokens in codas.

As shown in Table 11, deletion of /t,d/ in coda position was most likely when preceded by a nonhomorganic obstruent, /f, p, k/. Although the number of tokens in the environments /f/_ and /p/_ were low, the results were consistent with the patterning of /k/_. Deletion was next highest in the contexts /s/_ or /n/_, differing significantly from the highest group (/k/_ vs. /s/_; p = .04). Deletion was least frequent when the preceding consonant was a liquid. For comparison, Table 11 includes the deletion rate of coda /t,d/ after a vowel, which had a deletion rate between the environments with preceding liquids (p < .01) and /s, n/_ (p < .001).

Deletion by preceding consonant for tokens in codas (with vowel context included for comparison)

When we restricted our analysis to content words, vowel contexts did not differ significantly from liquid contexts. Preceding vowels and liquids patterned as a class, which we refer to as the class of approximants (Clements, 1990; Ladefoged,1982). (Because the glides /w, y/ did not occur before an alveolar stop in coda position in the data, /l, r/ are the only consonantal approximants in this context in the dataset.) Thus, tokens in codas had lower deletion rates when preceded by an approximant than when preceded by a nonapproximant. The class of nonapproximants may be divided further, with deletion more likely when the consonant is nonhomorganic. The results are summarized in (6).

The distinction between approximants and nonapproximants predicted deletion in the dataset (p < .001; β = 2.50), reflecting the fact that tokens preceded by nonapproximants had higher rates of deletion than those preceded by approximants. After controlling for this distinction, homorganicity of the preceding consonant also predicted deletion (p = .017; β = 1.03), reflecting the fact that deletion of a word-internal alveolar stop was more likely when preceded by a nonhomorganic consonant than when preceded by a homorganic consonant. After controlling for preceding consonant context using these two variables, there was no additional contribution to predicting deletion in coda tokens from preceding consonant bigram frequency.

Recall that previous studies found that the manner of articulation of a preceding consonant influenced final /t,d/ deletion. Obstruents (stops, fricatives) favored deletion but sonorant consonants, such as nasals and liquids, inhibited it. Our findings for coda /t,d/ are similar in that obstruents generally promoted deletion but liquids inhibited it. However, the classification of segments into the familiar manner categories (fricative, stop, nasal, liquid) is less revealing than the broader classification of sounds in terms of the categories approximant and homorganic.

Following consonant contexts

There were differences in the rate of /t,d/ deletion depending on the identity of the following consonant. Chi-square tests revealed a significant difference in the deletion rates between word-internal stops followed by /b, m, f, l/ and followed by /s/, as indicated in Table 12. However, there was no difference between the contexts _/b, m, f/, and _/l/. Similarly, no difference was revealed among _/s, n, v/. Deletion rates also differed significantly between _/s/ and _/z/. Deletion rates in the overall dataset thus suggested no classification of these following consonants based on phone features that might be used to predict deletion. That is, the environments _/b, m, f, l/ do not form a natural class, nor do _/s, n, v/. The deletion rate difference between _C onset tokens (or /t,d/ followed by a glide) and _C coda tokens (followed by a nonglide) were marginally significant by chi-square test. However, the distinction for following consonant of glide–nonglide did not predict deletion in the overall dataset or for tokens in onsets when this subset was tested separately.

Deletion rates by following consonant in onsets and codas

Our results are at odds with the results of previous studies of final /t,d/ deletion that found deletion to be negatively correlated with the sonority of the following segment, with less sonorous segments promoting deletion. In the present study, some of the highest rates of deletion occurred before a sonorant consonant (e.g., _m, 66.7%; _l, 50.5%). On the other hand, deletion before some obstruents was significantly lower (e.g., _s, 31.5%; _v, 23.1%; _z, 14.3%). Our results of internal /t,d/ deletion indicating that sonority is irrelevant in predicting deletion is in agreement with Guy (1991) and Labov (1995), who noted that the sonority of the following consonant in word-final /t,d/ deletion is insufficient to account for observed deletion patterns. Their alternative proposals were tested in the dataset.

Guy (1991) proposed that deletion of word-final /t,d/ is more likely when the stop is unable to syllabify with a following consonant. Our results for _/r/ and _/l/ are consistent with Guy's proposal. If the explanation is generalizable, then we would expect that word-internal alveolar stop deletion rates would pattern according to the syllable position of the preceding word-internal alveolar stop. Consonants that cannot be syllabified with a preceding /t,d/ include /b, f, m, l/, which are all in the group with the highest deletion rates in Table 12. However, /n, v/ also cannot be syllabified with a preceding /t,d/, and these two phones had lower deletion rates than the /b, f, m, l/ group. The phones /s, z, r, w/ will almost always (except for a very small number of cases with /s, z/, such as kids'll and whatsoever) syllabify with a preceding /t,d/. Because the groups /b, f, m, l, n, v/ and /s, z, r, w/ do not differ in their deletion rates, there is no relation between deletion rate and the ability of a consonant to syllabify with a preceding /t,d/.

In an alternative phonotactic account, Labov speculated that deletion rates may be tied to the frequency with which consonant cluster patterns are learned. Under the assumption that phone bigram frequencies in the lexicon reflect exposure of phone clusters to language learners during acquisition, then phone bigram frequencies of /t,d/ and the following consonant might be expected to predict deletion rates, with more deletion in higher frequency bigrams. Following consonant bigram frequency did predict deletion for /t,d/ for tokens in codas (p < .001; β = −.781) but not for tokens in onsets. The effect was limited to tokens in C_ contexts of codas (p = .029; β = −.600). The effect reflects the fact that alveolar stops in less frequent stop–consonant bigrams were more likely to delete than in more frequent bigrams; the mean following bigram frequency in C_ codas of deletions (5.02) was lower than for nondeleted /t,d/ (5.04). The direction of the effect is unexpected from Labov's proposal. Although we found no direct evidence that /t/ and /d/ behave differently in the contexts examined, the effect of following phone bigram frequency could provide evidence for a difference based on alveolar stop identity, because bigram frequencies are computed relative to the identity of the canonical /t/ or /d/ segment.

Finally, the difference in deletion rates between /s/ and /z/ shown in Table 12 might suggest that alveolar stop deletion is promoted by a following voiceless fricative. Note that generalizing the role of voicing to any following consonant would be problematic, because the highest rates of deletion are observed before the voiced segments /b, m, l/. Alternatively, because adjacent obstruents frequently share the same voicing property, the observation that more stops deleted before /s/ might be taken as synonymous with saying that more /t/'s deleted than /d/'s. Further analysis of the data revealed that neither explanation provides a straightforward account of the patterns. As shown in Table 13, although the proportion of deletion is greater for /d/ before voiceless consonants than voiced consonants, the observation that /t/ did not occur before voiced consonants makes it impossible to choose between the two accounts.

Deletion rates for /t/ and /d/ tokens by following sibilant context

Taking into account whether a consonant or vowel preceded the sequence is more elucidating. A /t/ or /d/ followed by a coronal fricative was more likely to delete when preceded by a consonant, regardless of which stop was at issue, as shown in Table 14. Conversely, when the sequence was preceded by a vowel, deletion of /d/ was rare, but it was more likely with /t/. Although this latter observation might suggest a fundamental difference between /t/ and /d/, on closer scrutiny, the cases in which /t/ deleted in this context were overwhelmingly function words, as shown in Table 14.

Deletion for /t/ and /d/ in sibilant contexts by preceding CV context

From these analyses, we may conclude that the higher deletion rate before /s/ is a consequence of both the part of speech and the preceding context. Specifically, when the stop-obstruent sequence is preceded by a vowel, deletion is higher when the /t,d/ is in a function word. This higher deletion rate in function words was particularly striking for /t/; the paucity of tokens with /d/ in this context made it impossible to determine whether /d/ deleted at greater rates in function words than in content words. Furthermore, when a coda /t,d/ was both followed by an obstruent and preceded by a consonant, deletion was highest. This finding is consistent with the results of the previous section, in that coda /t,d/ was more likely to delete when flanked by consonants.

Summary of phonological effects

Our results indicate a significant difference in the patterning of deletion in onset and coda tokens. When in onset position, an alveolar stop is more likely to delete when preceded by a vowel than by a consonant. Conversely, when the stop is in coda position, deletion is more likely when the stop is preceded by a consonant than by a vowel. After controlling for syllable position of the word-internal alveolar stop, whether the following segment was a consonant or a vowel was not significant, because of the fact that, by definition, stops in onsets are always followed by vowels (or glides in a small number of cases), and stops in codas are always followed by consonants. Thus, the overall finding that deletion was higher for coda stops than for onset stops is consistent with claims that deletion is more likely when a word-internal alveolar stop is followed by a consonant than by a vowel.

The effects of prominence were also different for tokens in onsets and codas. Tokens in the onsets of nonprominent syllables exhibited more deletion than tokens in prominent syllables. Tokens in the codas of nonprominent syllables also showed more deletion than those in prominent syllables, if they were in a V_ environment; however, if they were in a C_ environment, prominence enhanced deletion over nonprominence. These effects were only significant for tokens in content words, but tokens in functions words followed the same pattern, although the results were not significant, because of the small numbers of function words in many subsets.

When we examined the effects of individual vowels on deletion, we found that higher frequency vowel–stop bigrams promoted deletion of /t,d/ tokens in onsets, but not in codas. Stop–vowel bigram frequencies did not affect deletion in onsets or codas. Furthermore, no effects of the vowel classifications tested were found for vowels that preceded or followed word-internal alveolar stops. In particular, vowels with high off-glides had no effect on deletion of a following word-internal alveolar stop. However, there was a tendency for less deletion in flapping environments in which the stop is preceded by a high off-glide vowel, supporting the notion that high off-glides strengthen flaps, and thus make deletion in these words less likely.

When we examined individual consonants, deletion in syllable onsets was promoted when the stop was preceded by a coronal sonorant. This observation is expected if deletion and flapping are gradient outcomes of the same process, because this environment is largely the extended flapping environment V{nrl}_V⁰. For coda stops in C_ contexts, on the other hand, rates of deletion were higher when the word-internal stop was preceded by a nonapproximant than an approximant, and higher when it was preceded by a noncoronal than a coronal. After controlling for the quality of the preceding consonant context using these features, there was no contribution to predicting deletion from consonant–stop bigram frequencies in onset tokens or in coda tokens, confirming that it was the quality of a consonant, and not its likelihood in speech, that influenced deletion of a following alveolar stop. Moreover, deletion was more likely in contexts where consonant transitions involved changing the place of articulation, which encouraged cluster simplification.

There were no robust effects indicating that /t/ and /d/ exhibit different patterns of deletion, either in the overall dataset or in specific environments about which claims for differences based on stop identity have been made in the past. Higher rates of deletion of /d/ than /t/ in function words could be attributed to the behavior of subclasses of /d/-internal words (the indefinite pronouns, -n't contractions, and -'ve contractions) that exhibited higher than expected deletion rates given their phonological structure. Rhodes' claim that /d/ deletes more than /t/ in flapping environments was supported by higher rates of deletion of /d/ than of /t/ in the intervocalic flapping environment, although the difference was not significant, but in the extended flapping environment there was more deletion of /t/ than /d/. There was some support for Zue and Laferriere's (1979) claim that /t/ deletes more than /d/ in the nasal release environment, but only if we exclude the subclass of /d/ words with -n't contractions. With these words omitted, /t/ tends to delete more than /d/ in the nasal release environment, which may indicate a difference between /t/ and /d/ in nonlexicalized variation.

No features defining natural classes of consonants that followed word-internal alveolar stops predicted deletion of the preceding stop, although different consonants were associated with different rates of /t,d/ deletion. Deletion could also not be predicted based on the ability of an alveolar stop to resyllabify with a following consonant. However, less frequent alveolar stop–consonant bigrams promoted deletion for tokens in C_ contexts of syllable codas, accounting for some of the variation in deletion rates across individual following consonants in this one context. There was also evidence in some contexts that an effect of the following consonant was the result of an interaction of the following consonant's identity with the identity of, or class of, the segment preceding the alveolar stop. In short, no clear conclusions regarding the effects of different following consonants on word-internal alveolar stop deletion can be drawn from these results.