hi Gang
thanks so much for your response. the clips were randomized with respect to their order but had a fixed 16 s interval between them. there were 20 of one category and 10 of each of the others (these latter two types are the ones of most interest). as we are not interested in the statistics at the first level and were more interested just in the effect estimates I think this is fine--I can see why the residuals would impacted but I do not think this is a problem for us.
as for my second question in the original post, I will email you directly as it really isn't relevant to the board as a whole.
thanks again for your help.
James