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