“It is sometimes argued that we need rich qualitative data in order to find out not ‘what works’ but for whom and in what contexts. Anyone familiar with the design of experiments will agree. That question is answered by factorial designs with interaction terms… although main effects predominate in most educational datasets. If that last sentence seems like an unfamiliar concept, blame whoever taught you research methods and please make sure that your students are familiar with procedures that will underpin the thousands of controlled trials that education needs if it is to know that rather than simply guessing or asserting why.”
– Carol Taylor Fitz-Gibbon (2002). Knowing Why and Knowing That. Paper presented to the European Evaluation Conference in Seville, Spain. Emphasis and ellipsis in original.