Dear list I'm trying to test for differences in slopes of the 95% quantile regression fitted to two distinct subsets of my data set, as bellow: grx <- leaf.nitrogen gry <- leaf.photosynthesis ab1<- rq(gry[TRIP=="CAM"] ~ grx[TRIP=="CAM"],tau=.95) ab2<- rq(gry[TRIP=="WA"] ~ grx[TRIP=="WA"],tau=.95) When I try an anova(ab1,ab2), I get the following error message: Error in anova.rqlist(object, ...) : Models don't all have the same response variable I don't fully understand this and any tips on how to get around this? Thanks in advance, Tomas -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.