From: Adaikalavan Ramasamy <ramasamy_at_cancer.org.uk>

Date: Fri 01 Oct 2004 - 20:48:53 EST

Date: Fri 01 Oct 2004 - 20:48:53 EST

set.seed(1066)

x1 <- rnorm(50)

x2 <- rnorm(50)

Here are my own calculations that shows that perhaps the result when var.equal=TRUE is wrong.

n1 <- length(x1); n2 <- length(x2) m1 <- mean(x1) ; m2 <- mean(x2) ; num <- (m1 - m2) v1 <- var(x1) ; v2 <- var(x2)

# t-test with UNequal variance

denom1 <- sqrt( v1/n1 + v2/n2 )

num / denom1 # gives 0.5989774

# t-test with equal variance

sp <- ( (n1-1)*v1 + (n2-1)*v2 )/(n1 + n2 - 2)
denom2 <- sp * sqrt( 1/n1 + 1/n2 )

num / denom2 # gives 0.5913777

I tested this using R-1.9.1 (21/06/2004) on Redhat Fedora Core 2 and Windows 2000 Professional with the same results.

Any suggestions would be kindly appreciated.

Regards,

-- Adaikalavan Ramasamy ramasamy@cancer.org.uk Centre for Statistics in Medicine http://www.ihs.ox.ac.uk/csm/ Cancer Research UK Tel : 01865 226 677 Old Road Campus, Headington, Oxford Fax : 01865 226 962 ______________________________________________ R-devel@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-develReceived on Fri Oct 01 20:52:15 2004

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