In a research study aimed at understanding the relationship between caffeine consumption and cognitive performance, researchers designed a three-phase experiment.

In the first phase, 90 participants were randomly assigned to three groups: one group received a placebo, the second received a low dose of caffeine (50 mg), and the third received a high dose of caffeine (200 mg). After one hour, all participants completed a standardized cognitive assessment.

In the second phase, participants were asked to abstain from caffeine for one week, and then the same cognitive test was administered again, this time with all groups receiving a placebo to assess baseline performance.

In the final phase, all participants were re-assigned to their original groups but were given different cognitive tests that also measured reaction time and memory retention.

Which aspect of this experimental design is most crucial for ensuring the validity of the study's conclusions?