Dear R experts, I am trying to optimize my script, because right now it requires a lot of memory. The goal is to generate four plots in one page. Every plot corresponds to the means and sem's calculated for a given variable at different days. In order to obtain the means and sem's I apply the 'by' function. The way I have done it so far is like this: Read the data Generate a summary of the mean and sem of a variable at every Day. Plot the mean and sem of that variable. Repeat the same process for the other 3 variables. I tried to optimize the code by using a for loop, the code is below. #Reading the data dato<-read.csv('mydata.csv') names(dato)<-c("id","day","tx","var1","var2","var3","var4") dato<-dato[,1:7] #Specify varible to be plotted variable<-c('var1','var2','var3','var4') #Define parameters of window where panel: margins, number of plots in the panel windows(height=9, width=9, rescale='fixed') par(mfrow=c(2,2),xpd=T, bty='l', omi=c(0.8,0.25,1.2,0.15), mai=c(1.1,0.8,0.3,0.3)) for (k in variable) { dat<-dato[!is.na(k),] summ<-by(dat,dat[,c("tx","day")], function(x) { mn<-mean(x$k) std<-sd(x$k) n<-length(x$k) se<-std/sqrt(n) lowb<-mn-se upb<-mn+se data.frame(tx=x$tx[1],day=x$day[1],mn=mn,std=std,lowb=lowb,upb=upb,se=se) }) summ<-do.call("rbind",summ) #Definining x axis range xmax<-unique(max(summ$day,na.rm=TRUE)) xmin<-unique(min(summ$day,na.rm=TRUE)) yaxmin<-unique(min(summ$lowb)) yaxmax<-unique(max(summ$upb)) plot(1,1,type='n',xlab='Day',xlim=c(xmin,xmax),ylim=c(yaxmin,yaxmax), ylab=k, las=1,cex.lab=1,xaxp=c(xmin,xmax,diff(range(c(xmin,xmax))))) points(summ$day,summ$mn) } Where variable is a vector that specifies all the variables I want to plot. But I am getting the following error: ?Error in var(as.vector(x), na.rm = na.rm) : 'x' is empty In addition: Warning message: In mean.default(x$k) : argument is not numeric or logical: returning NA? Could some one please show me how to structure my code to achieve my final goal, which is to simplify it? I am attaching a csv file in case you want to run my code. Thank you very much in advance for your time and help, Judith ____________________________________________________________________________________ Be a better friend, newshound, and