1. How many subjects to run? Depends! Depends on: –What you are measuring –What question you are asking –How certain you want to be –Ability to get subjects

2. Examples Vision –Ss don’t vary much Use more when then do, e.g., with specticles, color blind –Measures may be noisy, so take more measures –understanding does not influence measures Learning –Ss vary –Measures are noisy, so take more measures –Can’t be reused because of learning Social aspects –Ss really vary –Measures are noisy, so take more iff possible –Can’t use yourself, theory says knowledge influences behavior

3. Examples Navigation of web site –If interested in learning, can be run once –If interested in leanring, use about 5 to 20 to 30 –Theory says, knoweldge of experiment may influence use, so can’t run yourself Choice of icons, do you like them? –Can only ask once –Theory of experiement influences behavior (you like me, you might like my choices and experiment) Choice of icons, can you ID them? Chice of icons, how fast can you read them? –Movie 2

4. More formal answers Provide context See what others do User Cohen’s Power calculations –Based on effect size (related to d’ !) If you knew, you would not be running experiment! See similar published papers Guess pilot –Likelihood of finding results If you care, make prob to.99, but these numbers are typically > 100 If you can live with small effects lost, then use medium to large effect sizes