Why is it that most psychophysics experiments only use a handful of subjects (typically around five, but sometimes even four, three or two), whereas the "norm" for experimental psychology (assuming a medium effect size) is around N=25? Is the motivation to reduce sampling error and increase external validity not the same for psychophysics as it is for e.g. decision-making experiments? Or is it the case that individual differences are smaller in perception than they are in cognition?


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It is partly as you already guessed, because the phenomena that are studied don't vary as much between individuals. This doesn't necessarily mean that individual differences in perception are smaller, it means that individual differences in those aspects of perception that are studied in psychophysics are smaller.

Another reason is that it is customary in psychophysics to instead use large numbers of stimuli - sometimes thousands - thus making sure that the estimate per individual is very precise. In that sense the more cognitive topics are much more underpowered, often employing groups of several tens of stimuli to generalize about entire classes of objects.

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    $\begingroup$ Many psychophysical tests show great variability within sessions and more so between sessions. A notorious example in my field are visual acuity measures in people with low vision, often showing large test-retest variability. $\endgroup$
    – AliceD
    Sep 30, 2016 at 19:47
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    $\begingroup$ Several perceptual properties (parameters), e.g. hearing/vision thresholds, are of course very heterogenous in the general population, so it still seems strange to me that a higher number of subjects is not used in the experiments used to say something about these properties. That way, the measurements would also have between-subject variability, whereas simply having many unique stimuli (or many repetitions) seems to me just increases the within-subject variability. $\endgroup$
    – z8080
    Oct 4, 2016 at 14:04

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