Any real surveys of sex workers?

[HaywoodJabloemy]

Have there been any surveys in Canada, like this one in Australia a few years ago? I can`t find any.
http://www.medicalnewstoday.com/releases/64277.php

Queensland`s female sex workers have similar levels of job satisfaction, physical health and mental well-being as women in the general population...
... Queensland University of Technology PhD researcher Charrlotte Seib... from the School of Public Health, surveyed 247 female sex workers aged between 18 and 57 years located throughout Queensland.

https://terb.cc/vbulletin/showthread.php?236039-The-Ask-A-Hooker-Thread/page36

... Accurate statistics have never been obtained.
Now perhaps if the workers were free from prosecution then they would come forward to answer academic surveys and then one would be able to gather accurate data on the industry as a whole ...

But is that really what is standing in the way? Most outcall escorts are not doing anything illegal, and even for others I doubt that answering a few survey questions from a university or some other (non-law-enforcement) organization could somehow lead to criminal charges.

Surely there could be some way to allow sex workers to anonymously take part in a survey. Would it really be that difficult to leave a bunch of questionnaires and postage prepaid envelopes at massage parlours and escort agencies that sex workers could complete and drop in a mailbox if they chose to participate? While this type of survey may not meet the strictest academic standards for establishing reliable statistics, I think it would be better than nothing, and may inform much of the general public about reality instead of the fictional nonsense from the prohibitionists.

Maybe it could be done simultaneously in different cities, and hopefully get at least as many responses as the 247 mentioned in the article about the Queensland Australia survey.