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> In a previous message from the list, someone asked about how to compare > a single subject with a population, to explore gray matter differences. > We are interested in to perform this analysis, exploring about possible > orbitofrontal cortical gray-matter hyponsity in our subjet. But he have > some questions. What is the better design Type? How many control > subjects we need to compare with the single subject? To explore > orbitofrontal cortex, a smoothing of 12 mm, is it correct? The more subjects you have, then the more sensitive the method. However, a comparison between a population and a single subject is not as sensitive to differences as a comparison between two populations. The number of control subjects you need will depend on how large the difference is compared to the amount of residual variance. The more controls you use, then the more sensitive the test is likely to be. However, in the case of a single subject comparison, the law of diminishing returns will kick in pretty soon. Even with an infinite number of controls, you would not necessarily see subtle differences. There are also issues about the validity of comparisons between single subjects and populations in terms of how normally distributed the data are. Unlike the case of a comparison between populations, the statistical tests are particularly susceptible to violations of the assumptions about normality. For this reason, it is necessary to smooth the data more to achieve suitably normally distributed errors. From the information I have available, I believe that 12mm smoothing may be enough. Best regards, -John -- Dr John Ashburner. Functional Imaging Lab., 12 Queen Square, London WC1N 3BG, UK. tel: +44 (0)20 78337491 or +44 (0)20 78373611 x4381 fax: +44 (0)20 78131420 http://www.fil.ion.ucl.ac.uk/~john