Try this
View(CO2)
ddply(CO2, .(Plant, Type, Treatment), summarize,
mean.conc = mean(conc,na.rm=TRUE),
mean.uptake = mean(uptake,na.rm=TRUE))
In your case
ddply(monvariable,.(Symbol,year,month), summarize,
mean.PE = mean(PE,na.rm=TRUE),
mean.PB = mean(PB,na.rm=TRUE),
mean.Turnover = mean(Turnover,na.rm=TRUE),
mean.beta = mean(beta,na.rm=TRUE)
)