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...DF and them comparing the two DFs based on this unique ID. Is there a better way? Many thanks for your help, Mark #compare two dataframes and extract uncommon rows #MAKE SOME DATA cars$id<-paste(cars$speed,cars$dist,sep="") $create unique ID field by pasting all columns together cars1<-cars[1:35,] cars2<-cars[16:50,] #EXTRACT UNIQUE ROWS cars1_unique<-cars1[cars1$id %in% setdiff(cars1$id,cars2$id),] #rows unique to cars1 (i.e., not in cars2) cars2_unique<-cars2[cars2$id %in% setdiff(cars2$id,cars1$id),] #rows unique to cars2

...= .....) Here is a simple (longer than necessary) example #### This modified from the last part of example(cars) {of R 1.5.1}: data(cars) ## An example of polynomial regression ## -- to show the bug, the linear example is sufficient cars.1 <- lm(dist ~ poly(speed, degree = 1), data = cars) cars1 <- lm(dist ~ speed, data = cars)# the `same' all.equal(predict(cars.1), predict(cars1))# the fitted values are ok cars.4 <- lm(dist ~ poly(speed, degree = 4), data = cars) newd <- data.frame(speed = d <- seq(0, 25, length = 51)) # shorter than in ?cars pr1d <-...