Translating Data

The logarithmic and square root transformations are typically most effective at normalizing data that have a minimum value near 1 and have a range that is at most a few orders of magnitude. If a variable consists entirely of large positive values, the transformed data do not show improved normality.

For example, if the minimum value of your data is $m$, you might want to subtract $m-1$ from your data as a first step so that the new minimum value is 1. You can translate (and scale) data by using the a+b*Y transformation in the Scaling/Translation family. Alternatively, the square root and logarithmic transformations are defined as log(Y+a) and sqrt(Y+a), so you can specify negative values for the $a$ parameter in these transformations. An example of this is presented in the next section.