Hi Jodi,
It sounds like the model is over-specified. Is rest or some other
baseline condition included in the design? If not, modeling all four
conditions will always lead to this error and there is basically nothing
that can be done given the design. On the other hand, the contrasts
comparing conditions (i.e. those that add to zero) are meaningful, even if
those that add to something other than zero are not.
Best,
Joe
|