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...lants nested within 17 stations. The data set is not balanced. Response variable is binary and repeated. I tried to fit this model model<- glmmPQL( y ~ z1.lon*lun + z2.lat*lun + z1.lon*lar + z2.lat*lar + z1.lon*sca + z2.lat*sca +z1.lon*eta + z2.lat*eta, random = ~ lun + lar + sca + eta | sta/piante, family=binomial, data=variabili) where y is presence (1) or absence (0) of a flowering lun, lar, sca, eta are level 1 variables z1.lon, z2.lat are level 2 variables. but during third iteration it stop because there is a singular matrix in solve. I stopped it after two iterations, however the...