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in my ANCOVA analysis, I have the case that my Y and X show a nonlinear pattern. Transforming either only Y or X does not help to stabilize variance. I did not find any hint that doing an ANCOVA in log-log space is illegal, however, I also did not find any note on that one should try out a transformation of Y AND X...

Could someone give me a hint if I am doing wrong with log-log?

thanks!

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Because ANCOVA is just a form of linear regression, you will find useful replies by searching on the regression and data-transformation tags. – whuber♦ Apr 11 '12 at 15:34
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Thanks, I found out that log(Y+2) already helped to stabilize the residuals. I used the logtrans function from the MASS package in R to come to this conclusion. – Jens Apr 12 '12 at 16:30

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I found that sometimes in necessary to "linearized" the relationship between X and Y. if you have an allometric relationship or for instance a power function, a log-log trasnformation help to do that and also ofcourse help to stabilize variances. it all depends in the nature of your data

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