In planning a clinical trial for demonstrating the efficacy of pioglitazone to resolve leucoplakia and erythroplakia in Fanconi anemia patients the need for a randomized controlled trial particularly under sample-size restrictions had to be discussed as very promising results were available from a single-arm clinical trial. The assumption of knowing the counterfactual is easily violated in single-arm trials and it is not possible to test it. Furthermore, undocumented and undetected patient selection is one of the main downfalls of single-arm trials. Contrary to the common opinion, addressing a new research question with single-arm trials requires more patients in the long run than starting with a (small) RCT from the beginning. Therefore, we contrast a single-arm based research strategy with a decision making strategy based on RCTs, which can be viewed (and used) as Lego®-type building blocks.