Hi Dimitris, you are describing a more stringent requirement than the t-test actually requires. It's the sampling distribution of the mean that should be normal, and this condition is addressed by the Central Limit Theorem.
Whether or not the CLT can be invoked depends on numerous factors, including the distribution of the sample, and the size of the sample, neither of which we have any information about. Liu, the problem you describe is associated with the application of the test rather than the test itself. The difference between log- and natural- scaled data can often profitably be thought about by asking whether you would naturally assume that the variation is additive (natural scale) or multiplicative (log scale). Given the information that you've presented there's no way we can tell which version of the test is more reliable. I hope that this helps. Andrew On Wed, Sep 22, 2004 at 10:00:16AM +0200, Dimitris Rizopoulos wrote: > Hi Liu, > > before applying a t-test (or any test) you should first check if the > assumptions of the test are supported by your data, i.e., in a t-test > x and y must be normally distributed. > > I hope it helps. > > Best, > Dimitris > > ---- > Dimitris Rizopoulos > Ph.D. Student > Biostatistical Centre > School of Public Health > Catholic University of Leuven > > Address: Kapucijnenvoer 35, Leuven, Belgium > Tel: +32/16/396887 > Fax: +32/16/337015 > Web: http://www.med.kuleuven.ac.be/biostat/ > http://www.student.kuleuven.ac.be/~m0390867/dimitris.htm > > > ----- Original Message ----- > From: "kan Liu" <[EMAIL PROTECTED]> > To: <[EMAIL PROTECTED]> > Sent: Wednesday, September 22, 2004 9:52 AM > Subject: [R] t test problem? > > > >Hello, > > > >I got two sets of data > >x=(124738, 128233, 85901, 33806, ...) > >y=(25292, 21877, 45498, 63973, ....) > >When I did a t test, I got two tail p-value = 0.117, which is not > >significantly different. > > > >If I changed x, y to log scale, and re-do the t test, I got two tail > >p-value = 0.042, which is significantly different. > > > >Now I got confused which one is correct. Any help would be very > >appreciated. > > > >Thanks, > >Liu > > > >__________________________________________________ > > > > > > > >[[alternative HTML version deleted]] > > > >______________________________________________ > >[EMAIL PROTECTED] mailing list > >https://stat.ethz.ch/mailman/listinfo/r-help > >PLEASE do read the posting guide! > >http://www.R-project.org/posting-guide.html > > > > ______________________________________________ > [EMAIL PROTECTED] mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide! > http://www.R-project.org/posting-guide.html -- Andrew Robinson Ph: 208 885 7115 Department of Forest Resources Fa: 208 885 6226 University of Idaho E : [EMAIL PROTECTED] PO Box 441133 W : http://www.uidaho.edu/~andrewr Moscow ID 83843 Or: http://www.biometrics.uidaho.edu No statement above necessarily represents my employer's opinion. ______________________________________________ [EMAIL PROTECTED] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html