From: Jim Brennan <jfbrennan_at_rogers.com>

Date: Fri 23 Jul 2004 - 09:35:31 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 09:47:48 2004

Date: Fri 23 Jul 2004 - 09:35:31 EST

Was just looking at this data and tried this:

*> event.var<--tapply(d.1$chem,factor(d.1$event),var)
**> event.var
*

1 2 3

-9.022233 -35.385033 -11.884433

Any idea why the variances are coming out negative.

> var(d.1$chem[1:3])

[1] 9.022233

right number ....

Anyway here is the answer I got if we assume independant means-- and I don't see why not to with the info given.

> sqrt(sum(s.err_event^2))

[1] 4.331732

(sum(event.var/3*-1)^.5)

[1] 4.331732

- Original Message ----- From: "Kenneth Cabrera" <krcabrer@epm.net.co> To: <jahernan@umn.edu>; <r-help@stat.math.ethz.ch> Sent: Thursday, July 22, 2004 7:27 PM Subject: Re: [R] Standard error of a sum

> If you supose that each mean is independent then

*>
**> s.err_total_chem <- sqrt(sum(s.err_event^2))
**>
**> If not you must estimate de var-covariance matrix to find this s.err.
**> This estimation will be difficult given the scarce data available.
**>
**> Hope it will helps.
**>
**> Kenneth
**>
**> 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 <-
**> >
**> >
**> >
**>
**> --
**> Kenneth Roy Cabrera Torres
**> Celular +57 (315) 504 9339
**>
**> ______________________________________________
**> 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

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 09:47:48 2004

*
This archive was generated by hypermail 2.1.8
: Fri 18 Mar 2005 - 02:37:31 EST
*