Then, do it again, but flip the groups so you can get within-subject measures.
You might expect that the hung-over students would perform worse on the tests. In surveys after the tests, those in the alcohol treatment certainly thought they'd done worse. But, there was no difference between the two groups' performance. The students thought that alcohol had impaired their performance, but there was no difference between the two.
While our findings are discordant with results of survey studies that find associations between alcohol use and academic problems, these studies are potentially confounded in that a third factor (e.g. personality) may cause both excessive drinking and academic difficulties and causal order is unknown (i.e. academic difficulties could lead to excessive drinking). Our findings are consistent, however, with a study on the effects of intoxication on next-day occupational performance . In that study, merchant marine cadets’ performance on a diesel engine simulator was not affected significantly, relative to placebo, on the morning after intoxication (mean BrAC.115 g%), but self-rated performance was significantly worse. Similarly, another laboratory study found measures of combined attention and reaction-time to be the only neurocognitive measures affected on the morning after 0.11 g% BrAC . [emphasis added]The Boston University story on the study is here; the full paper is here.
Now, what does this mean for WHO-approved survey measures of problem drinking that ask respondents how often their drinking kept them from doing what was normally expected of them? Might mean a bit of upward bias in responses there, mightn't it?