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EXISTING IMMIGRATION POLICY DATASETS AND THEIR LIMITATIONS
Hollifield and Wong (2013, 3) have argued that migration research in recent decades has "entrenched itself in the mainstream of political science." Developments in the field of immigration policy research are a very good example of this trend. After a long period in which studies that analysed single cases or a small number of countries predominated, a growing number of researchers have started to compare a relatively large range of cases. This has led to a quantification of the data under study and policy index building. By quantifying this data, migration scholars have followed a trend that has already taken place in other domains of political science such as democracy (Coppedge et al. 2011), statechurch relationship (Traunmüller 2012), citizenship (Bauböck and Helbling 2011), rule of law regulations (Skaaning 2010), and electoral systems (Teorell and Lindstedt 2010).
This article aims to give a short overview of recently compiled immigration policy indices and how the Immigration Policies in Comparison (IMPIC) dataset tries to overcome some of their limitations. Table 1 lists most of the existing databases and indices that measure immigration policies (Bjerre et al. 2015).2 It appears that with the exception of Timmer and Williams (1998), scholars only started to build policy indices just over a decade ago. Although a large number of important studies have already been published, several challenges are yet to be overcome in the field of immigration policy index building.
As far as temporal and spatial coverage is concerned, it becomes apparent that there is a trade-off between the span of time and the number of countries that are covered (Bjerre et al. 2015). For half of the indices, data were collected for one to three years only whereas the other half of indices allow for the analysis of longer periods. Several databases cover twelve years or more, allowing the investigation of developments across time (Givens and Luedtke 2005; Mayda and Patel 2004; Mayda 2004; Ortega and Peri 2009; Thielemann 2003; Timmer and Williams 1998). Peters (2014) has built an index that covers the immigration policies of 18 wealthy countries across four centuries. Three of the immigration policy indices cover a relatively large set of cases (Klugman and Pereira 2009; Ruhs 2011). The rest include...