Difficult to measure off-street prostitution

Today's Guardian reported a proposal from former cabinet minister Caroline Spelman MP for the purchase of sex to be criminalised.

The article quoted figures from a report by the Association of Chief Police Officers (ACPO) in 2010, which found that 17,000 of the estimated 30,000 women involved in off-street prostitution in England and Wales are migrants. Of these, it said, 2,600 were deemed to have been trafficked and a further 9,200 were viewed as vulnerable.

ACPO describe the figures as a "best estimate" and "an inferential analysis which draws on statistics where appropriate". Since the sector is hidden in nature, it's difficult to measure accurately.

The overall estimates were reached through regional research on businesses in the sector - from which the number of women working in each (and their nationality) was estimated. It wasn't comprehensive research, so where research was not conducted or where it was incomplete, the existing findings were extrapolated out.

The estimate of the thousands of those trafficked or vulnerable was based on a sample of just 254 women.

The House of Commons Library has summarised some of the context behind human trafficking.