Limitations in Experimental Design: Dietary Supplement Study

A research team aimed to investigate the impact of a specific dietary supplement on athletic performance. They recruited 40 athletes and divided them into two groups: one group received the supplement daily for eight weeks, while the other group received a placebo. The athletes' performance was measured using a standardized test of endurance at the beginning and end of the study period. Data collected included performance times and subjective reports of energy levels.

Which aspect of this experimental design is a potential limitation that researchers should address to improve the validity of their findings?