A randomized controlled trial evaluates a new omega-3 fatty acid supplement for cognitive enhancement in adults aged 60–75. Participants are randomly assigned to receive the supplement or an identical placebo for 6 months. Neuropsychologists who administer and score standardized cognitive function tests are aware of which participants received the active supplement versus placebo. At the end of the trial, the supplement group has significantly higher cognitive scores. Which type of bias is most likely influencing these results?