Applied linguistics is by definition an ‘applied’ discipline. As such, at the heart of virtually any aspect of applied linguistic research into second language acquisition (SLA) has been a concern to improve the quality of language learning. Accordingly, the ‘unit of analysis’ of such research has predominantly been the language learner – which, of course, left language teachers somewhat neglected: in most cases, they were seen as little more than one of the many factors affecting learner success. To make matters worse, when comparing the various factors affecting learning outcomes, language teachers have not fared particularly well, largely because it is rather difficult to pin down their individual impact on the success of their learners. This is well illustrated by the following summary concerning one specific area of language teacher psychology, namely, teacher motivation:

What is more, for quantitatively-oriented scholars, it is not only the manifold confounding variables that have hampered research efforts but also the logistical problem of generating a sufficiently large sample size of teachers willing to participate in order to ensure statistically significant results. When planning an ambitious research design for a quantitative study, a sample of at least 100 participants is typically seen as the bare minimum – or else one is likely to run into problems of insufficient normality of the distribution of the data. While this is usually easily achievable when investigating learners, the technicalities of producing a similarly large teacher sample may often be perceived as forbidding.