For the analysis of criminal activities, we made use of files of the Rotterdam- Rijnmond Police. The Ministry of Justice gave permission to use the data for this purpose.

Samples

The central sample has been taken from the files of the Aliens Department in Rotterdam (1989-1994). It is based on the VRS (Vreemdelingen Registratie Systeem), which has in the meantime been replaced by a new (national) system. The file contains information on apprehensions of 328 illegal immigrants. The sample fraction was 1:25. The central sample can be compared with two other samples, which contain data on legal foreigners who have been registered and apprehended in Rotterdam. These files were taken from (a) registration of legal aliens in VRS (sample fraction 1:150) and (b) registration of legally residing criminal suspects in HKS (Herkenningssysteem), sample fraction 1:40. A follow-up study was conducted in the four large cities in 1995. This study was based on a file of all apprehensions of illegal immigrants in the four largest cities in one year amounting to a total of nearly 7,000 apprehensions (Van der Leun et al. 1998).

Categorisation

The reasons for apprehension have been re-coded in order to differentiate between different types of criminal activities on the one hand and illegal residence or work on the other. When more reasons were mentioned, the decisive factor was the most severe reason for apprehension as registered in the police files. The Dutch legal system distinguishes between misdemeanours (overtredingen) and more severe offences (misdrijven). Offences were further classified into minor offences, serious offences and drug-related offences. Our five categories are based on this distinction, on the maximum sanction and on the type of legislation that is violated (see also Engbersen, Van der Leun and Willems 1995). In all instances, we had to rely on police reports. With the term ‘criminal activities’ we do not refer to the categories of illegal residence and misdemeanours. It must be noted that the data concern reasons for apprehensions, not convictions.