[R] Standard error of a sum

[Jose A. Hernandez]

Folks,

This is so simple is driving me crazy. It's not really an R question is 
more an Statistics question.

I applied a chemical in 3 different events during a growing season. I 
took 3 samples in each event to estimate mean applied chemical.

Thus, I can easily estimate means and st.err by event. I can then 
estimate the total chemical applied for the whole season by adding the 3 
means for each event. How can I estimate the standard error of this 
total applied chemical.

Thanks in advance,

Jose


event <- c(1,1,1,2,2,2,3,3,3)
chem <- c(131.75,125.92,130.09,42.67,48.26,54.56,85.05,91.17,90.86)
d.1 <- data.frame(event,chem)
attach(d.1)
s.err <- function(x) sd(x)/(sqrt(length(x)))

print(mean_event <- tapply(d.1$chem,factor(data.1$event),mean))
print(s.err_event <- tapply(d.1$chem,factor(data.1$event),s.err))

print(total_chem <- sum(mean_event))
s.err_total_chem <-



-- 
Jose A. Hernandez
Ph.D. Candidate
Precision Agriculture Center

Department of Soil, Water, and Climate
University of Minnesota
1991 Upper Buford Circle
St. Paul, MN 55108

Ph. (612) 625-0445, Fax. (612) 625-2208