[R] T-test syntax question
Adaikalavan Ramasamy
ramasamy at cancer.org.uk
Wed Nov 24 11:55:32 CET 2004
As Vito Ricci has already pointed out, the Welsh test is for two group
unpaired data with unequal variance assumption.
If you have the original data, say x and y, then you can simply do
t.test( x, y, paired=FALSE, var.equal=FALSE ).
If you do not have the original data, you can still calculate the
relevant statistics and p-value as long as you know the group length and
variance. 'stats:::t.test.default' shows you the code behind t-test. I
think the relevant bits are as follows
mx <- 0
my <- 2
mu <- 0
# You will need to fill these with your observed values
vy <- var(y)
vx <- var(x)
ny <- length(y)
ny <- length(y)
stderrx <- sqrt(vx/nx)
stderry <- sqrt(vy/ny)
stderr <- sqrt(stderrx^2 + stderry^2)
df <- stderr^4/(stderrx^4/(nx - 1) + stderry^4/(ny - 1))
tstat <- (mx - my - mu)/stderr
# for two sided alternative
pval <- 2 * pt(-abs(tstat), df)
alpha <- 1 - conf.level
cint <- qt(1 - alpha/2, df)
cint <- tstat + c(-cint, cint)
cint <- mu + cint * stderr
On Wed, 2004-11-24 at 04:28, Steve Freeman wrote:
> Hi.
>
> I'd like to do a t-test to compare the Delta values of items with Crit=1
> with Delta values of items with Crit=0. What is the t.test syntax?
>
> It should produce a result like this below (I can't get in touch with the
> person who originally did this for me)
>
> Welch Two Sample t-test
>
> data: t1$Delta by Crit
> t = -3.4105, df = 8.674, p-value = 0.008173 alternative hypothesis: true
> difference in means is not equal to 0
> 95 percent confidence interval:
> -0.04506155 -0.00899827
> sample estimates:
> mean in group FALSE mean in group TRUE
> 0.03331391 0.06034382
>
> Thanks.
>
> ------------------------------------
> Steven F. Freeman * Center for Organizational Dynamics * University of
> Pennsylvania * (215) 898-6967 * Fax: (215) 898-8934 * Cell: (215) 802-4680 *
> stfreema at sas.upenn.edu * http://center.grad.upenn.edu/faculty/freeman.html
>
>
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>
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--
Adaikalavan Ramasamy ramasamy at 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
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