Criminal phenomena such as forced labour present obvious measuring challenges. Administrative statistics of recorded cases present us with just the tip of the iceberg. Global estimates vary from 20.9 million to 45.8 million people who fall victim to some form of modern slavery.

The international fight against forced labour goes back as far as 1930 with the adoption of the Forced Labour Convention in that year. In 2015 the United Nations adopted a programme of Sustainable Development Goals (SDG’s) which includes goal-setting in the fight against forced labour/human trafficking/ modern slavery. The SDG’s is accompanied by agreement on a joint UN programme to collect statistics to monitor progress. Member states are requested to regularly collect statistics on the number of victims of forced labour/human trafficking with breakdowns according to gender, age and type of exploitation.

Measuring methods forced labour

It was on prof. J. van Dijk’s initiative that DSP-groep tested various measuring methods for collecting data in a small pilot study among Bulgarian migrant workers in the Netherlands. The International Labour Organization in Geneva co-funded this project. For this project DSP-groep collaborated with Fair Work in Amsterdam. Fair Work is an independent non-gouvernmental organisation (NGO) that supports women and men in exploitative situations in the Netherlands.

With this study we wanted to

  • field test the feasibility of a respondent-driven sampling procedure, modelled after the study of prof. S. Zhang;
  • field test a questionnaire designed to measure the prevalence of forced labour;
  • explore whether or not the survey method can provide insights in the nature of labour exploitation, including in the sectors most involved in exploitative practices.


The respondent-driven sampling method proved to be partly successful. Although it provided us with 64% of new respondents, people not known to us beforehand, the chains of respondents were insufficiently long to obtain randomization.  The questionnaire proved to be successful and the interviews provided us with valuable information on the situation of Bulgarian migrant workers.

However, the prevalence of forced labour in the Netherland and elsewhere is still relatively unknown and further research is needed. The results will feed into the ongoing research efforts of the ILO and UNODC.

Read about our international study on the new phenomena of labour exploitation.

Would you like to know more?

Feel free to contact Manja Abraham.