I want to compare evolution rate between two set of data (morphological and reproductive traits) because the values of the reproductive traits is very small in compare with morphological traits (both of the same unit and scale [mm]), so I want to perform a log-log regression of species mean trait measurements on species mean thorax volume which I used it as the index of the body size, but I don't know how can I perform it. Thus, I am looking for help or potential collaborator.
thanks for any suggestion.
First, it's not really 'evolution' rates you are measuring, but rather 'variability' of species.
Second, you should do a correlation rather than a regression imo because you can't know which variable, if any, is dependent.
Third, you should only use log transformation if you think the relationship is non-linear or skewed and you want want to preserve the linear model.
Fourth, you need to test the residuals using a plot: a residual plot is a graph that shows the residuals on the vertical axis and any variable on the horizontal axis. If the points in a residual plot are randomly dispersed around the horizontal axis, a linear model is appropriate for the data; otherwise, a non-linear model is more appropriate.
BTW: the difference between the observed value of the variable and the predicted value is called the residual.
I hope this helps :)
Transformation does not LEAD to skewed data; rather, transformation changes skewed data to normal :)
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