This chapter presents summaries of the methodology and results of studies relevant to SDA in naturalistic contexts. The primary aim is to describe how much of the D2 was acquired by individuals and groups of subjects. As in studies of SLA, the particular degree of acquisition is referred to here as the level of attainment. The first section of the chapter deals with the question of how attainment in SDA can be measured. The following two sections outline seventeen studies of SDA, eight done on dialects of English and nine on dialects of other languages. After a summary of the overall results, there are brief descriptions of three other studies that focus on shift from the D1 rather than attainment in the D2. The chapter concludes with a discussion of some methodological issues.

Measuring attainment in SDA

How can we measure attainment in SDA? Because of the linguistic similarities that usually exist between the D1 and D2, many of the methods used in SLA studies are usually not suitable – for example, grammaticality judgements and measures of fluency and communicative success. This is because D1 speakers normally know the basic grammar of the D2, since it is virtually the same as that of the D1. Furthermore, because of mutual intelligibility, they can usually communicate fluently by using the D1 with D2 speakers. That leaves us with examining observable behaviour – i.e. performance data. More specifically, we need to concentrate on linguistic features that distinguish the D2 from the D1.