How many subjects constitute a study?

In fMRI there are two classes of inference: one aims to make a comment abou t the "typical" characteristics of a population, and the other about "avera ge" characteristics. The first pertains to studies of normal subjects that try to identify some qualitative aspect of normal functional anatomy. The s econd class necessarily applies to clinical neuroscience studies that want to make an inference about quantitative differences of a regionally specifi c nature. The first class of inferences is adequately serviced by conjuncti on analyses and fixed-effects models with relatively small numbers of subje cts. The second requires random-effect analyses and larger cohorts. (C) 199 9 Academic Press.