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2. The Measurement of Modern Slavery – Operational Challenges

Estimations of trafficking and slavery are made more difficult by the ‘indeterminate duration’

of the crime


The estimation of the ‘dark figure’ for any crime primarily rests on the ability to conduct

random sample crime surveys.


For the purpose of victimisation surveys, it is assumed that most crimes are discrete, time-

bound events of relatively short duration.


Slavery, however, is more a process than an event; it is an open-ended victimisation. In one

legal definition it is a ‘crime of indeterminate duration’.


This has generated the paradox that while the indeterminate length of enslavement over time

is included in its definition, it also goes unacknowledged as to its effect on the estimation of

slavery within criminal statistics.

Multiple Systems Estimation (MSE) is an effective technique for estimating levels of slavery and

can be applied at city, country and regional level


One answer to this measurement challenge came with the application of Multiple Systems

Estimation (MSE) to modern slavery crime in the UK. This technique has previously been used

for a number of hard to reach populations from fish stocks to illegal drug users to the victims of

mass atrocities in war.


The successful identification of a ‘dark figure’ for slavery crime in the UK, along with an

estimate of the total number of victims, has meant that MSE might be applied in other settings

where slavery occurs.


We find that MSE estimation technique can be used at different levels. While the first test of

the method was for the UK, we find it can also be used at the level of a single city, county, or


There is a need to provide better guidance and instruction to Governments in the use of MSE

and other statistical approaches to the quantification of modern slavery


What is now lacking is a clear dissemination of the technique in both popular and detailed

methodological formats. Governments and other administrative units need transparent and

useful descriptions and instructions to bring this technique to broad adoption thus generating

reliable estimates of slavery crime across populations.


MSE is only one possible approach, and there is a clear need for further research and

information sharing to allow the development of more granular approaches to the

quantification and understanding of Modern Slavery. A crucial aspect is a change of culture

towards open data and reproducible research and methodology. Serious thought needs to be

given to ways of doing this without compromising ongoing investigations and the anonymity of

victims (and of possible perpetrators).