Thanks to the help I have received from Uwe and Dimitris I have been able to simulate the data as a list rather than separate objects and structure in the long format. For the most part, all of my code is working properly. There is one issue that I can't resolve. When I do the following all seems to work as expected lapply(long, function(x) summary(x$score.1)) #This works Or lapply(long, function(x) lm(x$score.1~x$time) #This works But, when I replace the lm() with gls() I receive the following error gltest<-lapply(long, function(x) gls(x$score.1~x$time, correlation=corAR1(form=~1|x$V1), method='ML')) Error in eval(expr, envir, enclos) : Object "V1" not found If I remove the correlation argument altogether I then get an error that score.1 is not found. Why is it that my variables are "found" in other situations but not for the gls call? Thank you, Harold [[alternative HTML version deleted]]