From: Spencer Graves <spencer.graves_at_pdf.com>

Date: Fri 23 Jul 2004 - 07:00:28 EST

R-help@stat.math.ethz.ch mailing list

https://www.stat.math.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Fri Jul 23 07:06:31 2004

Date: Fri 23 Jul 2004 - 07:00:28 EST

The variance of a sum is the sum of all the elements of the variance-covariance matrix. If the summands are uncorrelated, then the variance of the sum is the sum of the variances. hope this helps. spencer graves

Jose A. Hernandez wrote:

> 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 <-
**>
**>
**>
*

R-help@stat.math.ethz.ch mailing list

https://www.stat.math.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Fri Jul 23 07:06:31 2004

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