data(greatLakesM)
mErie <- ts(greatLakesM[,'Erie'], start=1918, frequency=12)
greatLakes <- aggregate(greatLakesM[,-(1:2)], by=list(greatLakesM$year),
FUN=mean)
names(greatLakes)[1] <- 'year'
## maybe str(greatLakesM)
Run the code above in your browser using DataLab