if you are talking about a t test for means ... most software would
automatically give a two tailed p value ... unless you specify otherwise
(which software usually will let you do)
here is the typical example
Two-sample T for C1 vs C2
N Mean StDev SE Mean
C1 10 25.70 2.87 0.91
C2 10 27.50 3.66 1.2
Difference = mu C1 - mu C2
Estimate for difference: -1.80
95% CI for difference: (-4.90, 1.30)
T-Test of difference = 0 (vs not =): T-Value = -1.22 P-Value .238
when ns are 10 for each ... df would be 18 for the two sample t
(approximately) ... so, here is what a t distribution with df=18 looks like
:: :
.:.::.:::.
. :::::::::::::...
::::::::::::::::::: .
. .. .....::::::::::::::::::::::::::::::... .. . . .
---+---------+---------+---------+---------+---------+---C3
-3.0 -1.5 0.0 1.5 3.0 4.5
the p value of .238 is figured in the following way:
from 0 ... go to the negative side to -1.22 ... and also from 0 to the
right side to +1.22 ... and find the area BELOW -1.22 and ABOVE +1.22 ...
this is the p value of .238 that gets printed out ...
two tails ...
At 11:25 AM 4/4/01 -0500, auda wrote:
>Hi,
>What is the p-value of a t-statistic significant (significant level shown by
>the software is p) in the wrong direction in an one-tailed test? Should we
>modified it to (1-p)? Or it is just p?
>
>
>Erik
>
>
>
>
>=================================================================
>Instructions for joining and leaving this list and remarks about
>the problem of INAPPROPRIATE MESSAGES are available at
> http://jse.stat.ncsu.edu/
>=================================================================
_________________________________________________________
dennis roberts, educational psychology, penn state university
208 cedar, AC 8148632401, mailto:[EMAIL PROTECTED]
http://roberts.ed.psu.edu/users/droberts/drober~1.htm
=================================================================
Instructions for joining and leaving this list and remarks about
the problem of INAPPROPRIATE MESSAGES are available at
http://jse.stat.ncsu.edu/
=================================================================
