An experiment was designed to investigate the effect of different dosages of a new sleep aid on the quality of sleep among adults. The researchers randomly divided 120 participants into four groups: one group received a low dosage, another group received a medium dosage, the third group received a high dosage, and the final group received a placebo. Each participant was monitored using a sleep tracker for one week. At the end of the week, participants rated their sleep quality on a scale of 1 to 10.

Based on the design of this study, which of the following aspects best demonstrates a flaw in the experimental design?