'Quantities'

Logarithmic transformation can only be used for data sets consisting of positive values — logarithms are undefined for negative or zero values. They are therefore particularly useful for quantities — i.e. amounts of something. Indeed, many researchers routinely apply logarithmic transformation to quantity data before analysis.

When are they effective?

A log transformation affects the shape of the distribution most when the ratio of the largest to the smallest value in the data is large. When this ratio is less than 10 (one order of magnitude) then the transformation has much less influence on the shape of the distribution, as in the data set below.