"Empirical studies are extremely difficult to conduct in applied linguistics, as indeed in any other area of social sciences or the humanities, given the enormous number of different factors which can impact on any given study" (p.264).
Boulton, A. (2012). Corpus consultation for ESP. In Boulton, A., Carter-Thomas, S. and Rowley-Jolivet, E. (eds.). 2012. Corpus-Informed Research and Learning in ESP. Issues and applications. Studies in Corpus Linguistics, 52. Amsterdam: John Benjamins: pp. 261-292.