[1] A variable rules analysis is designed to provide a quantitative model of a situation where speakers alternate between different forms that have the same meaning and stand in free variation, but in such a way that the probability of choice of either the one or the other form is conditioned by a variety of context factors or social characteristics.
A variable rules analysis computes a multivariate statistical model, on the basis of observed token counts, such that each determining factor is assigned a numerical factor weight that describes how it influences the probabilities of choice of either form.
Although the necessary computations required for a variable rules analysis can be carried out with the help of mainstream general-purpose statistics software packages such as SPSS, it is more often[needs update] done by means of a specialised software dedicated to the needs of linguists, called Varbrul.
It was originally written by David Sankoff and currently exists in freeware implementations for Mac OS and Microsoft Windows, under the title of Goldvarb X.
[3] Variable rules approaches are commonly employed for the analysis of data in sociolinguistic research, especially in studies that aim to investigate how reflexes of linguistic change through time appear in the shape of structured variation patterns within a speech community.