[R] t test problem?

Andrew Robinson andrewr at uidaho.edu
Wed Sep 22 11:34:48 CEST 2004

> Hi, Many thanks for your helpful comments and suggestions. 

You're welcome.

> The attached are the data in both log10 scale and original scale. It
> would be very grateful if you could suggest which version of test
> should be used.

I feel that it would be inappropriate.  It depends on the origin
of the data.  You, as the analyst, must make that decision.  If you're
analyzing the data for someone else, then you should make that
decision with their input, or have them make it.

> By the way, how to check whether the variation is additive (natural
> scale) or multiplicative (log scale) in R? 

Unfortuantely you can't do that.  It depends on the context of the
data, which is not amenable to testing.

> How to check whether the distribution of the data is normal?

Tests exist for this, but there are problems with their usage.  Try
searching the help or your preferred search engine for more information.

> PS, Can I confirm that do your suggestions mean that in order to
> check whether there is a difference between x and y in terms of mean
> I need check the distribution of x and that of y in both natual and
> log scales and to see which present normal distribution? 

No, I don't agree with that statement, and I don't think that it
reflects my earlier message.

> and then perform a t test using the data scale which presents normal
> distribution?

Again, I don't agree.  I would advise you to apply the appropriate
test (maybe a t test, maybe not) to the data on the scale that is
suggested by the sampling scheme, the origin of the data, and the
hypothesis that interests you.

I hope that this helps.

Andrew Robinson                      Ph: 208 885 7115
Department of Forest Resources       Fa: 208 885 6226
University of Idaho                  E : andrewr at uidaho.edu
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.

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