I am confused about the mixed advice regarding controlling for baseline differences.
Would you always control for a baseline between groups difference on a particular variable or only if the variable correlates with the DV?
I am using SPSS and conducting Mixed Model analyses to evaluate an intervention.
Jeromy, I tried to answer by adding comment, but can't seem to make it work. To answer:
@jeromy-anglim: Well it is a group randomised design (schools randomised into intervention or waitlist control). However, participants from waitlist control schools (control condition) were sent an invitation to take part in a questionnaire study, whereas participants from the intervention schools were invited to take part in a parenting intervention. Hence, a slightly more distressed sample (despite efforts to avoid this by offering child participants a $30 voucher). In my case there are more boys in the intervention sample, and the intervention sample is on average 3 months younger. No other baseline differences for adolescent report. But baseline differences on almost all parent reported outcome variables (with the intervention group being more distressed).