Published studies of linguistic variation usually concentrate
on the main effects apparent for linguistic and social factor
groups. By contrast, interaction effects, whereby the influence
of some factor(s) is conditional on the values of other factor groups,
have received considerably less attention and, even when recognized,
are rarely quantified. Interaction effects involving linguistic factor
groups are especially rarely acknowledged, even though the existence of
interactions between social factors is widely recognized. This article
reclaims interaction effects as an object of systematic variationist
study and demonstrates the benefits of including such effects in
quantitative modelling: first, by outlining practical methods for
investigation of interaction effects within variable-rule analysis;
second, by providing direct evidence for the incidence of interaction
effects in linguistic data, through reanalyzing several pre-existing
studies of phonological variation containing both linguistic and
extralinguistic factor groups; and finally, by discussing their
interpretation.This article is the
result of many years of collaboration with Janet Holmes, who generously
provided all of the datasets reanalyzed here. Thanks are also due to
David Britain, Peter Patrick, Tom Veatch, and John Paolillo for advice
and correspondence on goldvarb/varbrul at various stages
over the past decade. The present version has greatly benefitted from
audience feedback at the 14th New Zealand Linguistic Society Conference
(Christchurch, August 2001) and from the detailed comments of the
anonymous referees for LVC.