[FSL] [TitleIndex] [WordIndex

When do I demean (mean center) an EV?

If your EV is a covariate (of interest or not interest) then it is almost always advisable to demean this. Sometimes the results of particular contrasts do not depend on whether you demean or not (so not demeaning may not be "wrong" but it does not hurt). In other cases it is important to demean or the interpretability of the results is lost. If you have more than one group in your design then it is generally the case that you should demean across all groups and not within each group, however exceptions to this do exist and you should discuss this with someone with expertise in statistics if you are unsure.

For more on exactly what happens when you center and when it is or is not important, see Jeanette Mumford's guide on Mean centering continuous covariates for a group fMRI analysis.

What happens if I want to estimate a differential [1 -1] contrast between two conditions that have very different numbers of events than each other - is this valid?

This is valid, because the estimated BOLD effect size ("PE") for each condition is not biased by the number of events, it just has different uncertainty for the two conditions (because of the different numbers of events in each). Hence the COPE (contrast between the two BOLD effect sizes) will also be unbiased. The uncertainty on the COPE (which affects the size of the related Z-stat) will be affected by the different numbers of events. In general the Z-stat will be maximised when the numbers are the same, but in all cases it will still be valid.


CategoryFaq


2013-09-30 13:48