Adrianus Dingeman de Groot (1914–2006) was one of the most influential Dutch psychologists. He became famous for his work “Thought and Choice in Chess”, but his main contribution was methodological — De Groot co-founded the Department of Psychological Methods at the University of Amsterdam (together with R. F. van Naerssen), founded one of the leading testing and assessment companies (CITO), and wrote the monograph “Methodology” that centers on the empirical-scientific cycle: observation–induction–deduction–testing–evaluation. Here we translate one of De Groot's early articles, published in 1956 in the Dutch journal Nederlands Tijdschrift voor de Psychologie en Haar Grensgebieden. This article is more topical now than it was almost 60 years ago. De Groot stresses the difference between exploratory and confirmatory (“hypothesis testing”) research and argues that statistical inference is only sensible for the latter: “One ‘is allowed’ to apply statistical tests in exploratory research, just as long as one realizes that they do not have evidential impact”. De Groot may have also been one of the first psychologists to argue explicitly for preregistration of experiments and the associated plan of statistical analysis. The appendix provides annotations that connect De Groot's arguments to the current-day debate on transparency and reproducibility in psychological science.