åRandom models:
íTesting for differences in variances
between factors
íConsiders levels of the
random factor as a random sample from a larger population. Hypothesis testing of
the random effect can thus be extended to entire population.
íObviously, one cannot always
use random effect model (consider the Ôdrug typeÕ factor)
íSubjects are often used as a
random factor
åRandom model tests yield
lower F-statistics (less statistical power) because variance of factor
effects is tested against that of both factor means, which is often larger
than the error variance used in fixed effects.
íThis is better expressed in
the equations of F-ratios that we avoided using in this presentation
åIntermediate effects (mean and
variance differences) would be nice
íNot a standard statistical
formula, and not available in AFNI yet
¥