alkfos<-read.csv("alkfos.csv")
alkfos.pctchange<-(sweep(alkfos[-1],1,alkfos$c0,"/")-1)*100
available<-aggregate(!is.na(alkfos[-1]),list(alkfos$grp),sum)
means<-aggregate(alkfos.pctchange,list(alkfos$grp),mean,na.rm=T)
sds<-aggregate(alkfos.pctchange,list(alkfos$grp),sd,na.rm=T)
available<-as.matrix(available[-1])
means<-as.matrix(means[-1])
sds<-as.matrix(sds[-1])

sems<-sds/sqrt(available)

times<-c(0,3,6,9,12,18,24)