Utilized a concurrent discrimination paradigm with 17 Ss in 2 experiments to study the effects of visual attention on psychophysical judgments and the consistency of these effects with a sample-size model in which attention influences the variance of the internal representation used to make psychophysical judgments. In Exp 1, the relationship between attention and discrimination accuracy was consistent with the sample-size model when attentional allocation varied from trial to trial and when it varied between blocks. Discriminations were more accurate overall with varied than with blocked attentional allocation. Besides supporting the sample-size model, the results of Exp 2 suggest that criterion variance is at least as large as sensory variance and that criterion but not sensory variance increases with stimulus variance.