[R] Standard error of a sum

From: Jose A. Hernandez <jahernan_at_umn.edu>
Date: Fri 23 Jul 2004 - 06:50:42 EST


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,


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)
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

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Received on Fri Jul 23 06:56:52 2004

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