Please, help with a formula for dealing with unbalanced design: To see the counts: aggregate(dfa$CertId,by=list(type=dfa$ComType,stat=dfa$StatusCodeId),length) type stat x 1 C 1 6571 2 C 3 28957 3 C 8 12390 4 C 11 12415 5 E 13 9 6 R 13 51 7 E 15 2079 8 R 15 6692 I would like to have a slope for statuses 1,3,8,11,13 and two slopes for status 15 one for type E and one for type R. I tried nesting, but it assumes that all levels exist for each factor and complains about singular model matrix. Is there a theoretically proper way to deal with this or I should just relabel status 15 and make it 16 for type R and regress on status alone?? Thanks everybody Stephen B