--- Song <[email protected]> wrote:
> I need to use log transformation before regression analysis. Because
> some of my variables contain 0, I need to use log (1+x) instead of
> log (x), where x contains only positive values. My question is... Do
> I need to use log (1+x) if x does not contain 0, but contains values
> less than one? I think I can use log (x) in this case.
This question comes up so every once in a while, and the answer is
always that you should not do the log(1+x) transformation at all. If
the variable is your dependent variable you can avoid doing that by
using -glm- with the -link(log)- option. If your variable is an
independent variable, the concern is linearity of the effect of that
variable, so you can work around that by entering it as a spline. See:
-help glm- and -help mkspline-.
Hope this helps,
Maarten
-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology
Vrije Universiteit Amsterdam
Boelelaan 1081
1081 HV Amsterdam
The Netherlands
visiting address:
Buitenveldertselaan 3 (Metropolitan), room Z434
+31 20 5986715
http://home.fsw.vu.nl/m.buis/
-----------------------------------------
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