The importance of interaction effects

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.