[R] Questions of t.test {stats}

(Ted Harding) Ted.Harding at nessie.mcc.ac.uk
Tue Oct 19 09:31:10 CEST 2004

On 19-Oct-04 pcscan wrote:
> We are currently using the t-test in Package stats,
> [...]
> but have some troubles :
> 1. why does the t-test take so a long time to perform a single test
>    on a row of a data.frame ? Is there any alternative function to
>    perform t-test on all the rows of a data.frame ?

Not sure what your problem is here. 't.test' is almost instantaneous
on my somewhat slow machine here.

> 2. We got different results on the following data with the argument
>    var.equal  setting as TRUE and FALSE respectively.
>    We are curious why the  "Welch Two Sample t-test" couldn't
>    distinguish these two vectors well.

The two samples have very different standard deviations, the SD of y
being about 18 times that of x. You would expect the Welch test to
give quite different results from the "var.equal=TRUE" case. In these
circumstances I would trust the Welch test rather than the other.

Best wishes,

E-Mail: (Ted Harding) <Ted.Harding at nessie.mcc.ac.uk>
Fax-to-email: +44 (0)870 094 0861  [NB: New number!]
Date: 19-Oct-04                                       Time: 08:27:01
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