After waiting for almost two weeks, finally we have something that looks like the source of the claim of Jojanneke that '70% of the window prostitutes are forced'. Strangely enough we found the (what seems to be likely) source not because the Public Prosecution Office was able to give us the source.
No, the (likely) source was not provided by the Public Prosecution Office, who know the numbers so well, except where they come from. The source was provided by Marieke Schenk, who contacted my boyfriend after she had suspicions that this might be the source.
So far this source is the only one we have that fits the description of a number being mentioned of 70% by the Public Prosecution Office in relation to trafficking numbers. Strangely however this source nearly mentions the number of 70% and the two reports on what they based it on, but does not give us the source of how they came to this conclusion. I'm therefore afraid that the number of 70% is nothing more than near guesswork of Werner ten Kate, rather than actual research.

The source is actually a a report to do a quick scan to give politicians an idea about what the results could be, and what the problems are, regarding the new regulations they want to implement for prostitution. This report is not a research about how many girls are forced or trafficked.
The number of 70% is also nearly mentioned in the report, it is not the conclusion of the report itself, and can therefore be considered as nothing more than the personal opinion of the person who stated it, Warner ten Kate himself. If there is however some source that proves it are not just his personal guessing, but an actual research, I'd sure like to see it.

The page on which the number gets mentioned is page number 102. There you will find an interview by the Nationaal Rapporteur with Werner ten Kate from the Public Prosecution Office and Kai Lindenberg from the University of Groningen (who is also the writer of the entire rapport himself). During an interview with Eva Jinek, Jojanneke clearly stated that these numbers are based upon reports like KLPD's Schone Schijn and research of the Erasmus University. This source however does not mention the Erasmus University at all. The line we are talking about is the following:

"Ondertussen is het wel zo dat onderzoeken die bij de zaken Sneep en Koolvis zijn uitgevoerd en die juist op het legale circuit zien, uitwijzen dat circa 70% van de vrouwen in dat legale circuit gedwongen werkt."

Translation:
"Meanwhile it is the case that researches that were done in the Sneep and Koolvis case that focused on legal prostitution, show that about 70% of the women in the legal prostitution are forced to work."

In other words, this is not a research that proves that 70% is forced, it is nearly a report that mentions someone's personal estimations. A trick we've seen before. Because, if you look at what Werner ten Kate is referring to, you'll be very shocked!

The first rapport that gets mentioned is the Sneep case, that rapport is now known as the KLPD Schone Schijn rapport, which mentions a number of 50-90% forced prostitution and/or exploitation. In other words, not just forced prostitution, but they count in exploitation as well, quite correctly, since those are the two things that are called human trafficking together. So far so good.
The 50-90% however is not based on any actual research. It is based upon the personal opinions of nearly 6 police officers, who made estimations ranging from 50 to 85% (page 76), from the total of 18 police offers that were interviewed for the rapport (page 134), The other police officers stated that this was simply impossible to give a good estimation. In short, this is nearly the opinion of 33% of the interviewed police officers.
Fact also is that the 50-90% isn't even correct, it should have been 50-85%. But besides that, they are nearly people's personal opinions and guessing, not based on any scientific research. The report itself never did any type of research to confirm or deny if the estimations of the individual police officers give as a good estimation or not. No prostitute was asked to give any estimations, nearly the a few police officers and their own guesswork. Nothing more. I hardly call that a reliable number.

The second case that Werner ten Kate mentions as a source, is the Koolvis case. This was a large case of a Nigerian gang that used Holland as a gateway into Europe, to force 133 women into prostitution in Italy and Spain. That's right! These girls didn't work as prostitutes in Holland, they were nearly taken from asylum centers in Holland, to be transported to other countries to work in prostitution over there.
In short, these girls didn't work in Holland in prostitution, but in other countries. So it says nothing about how many girls are forced or exploited in Holland. How one can draw a conclusion about how many women are forced in Holland in prostitution, based on a case that's about trafficking girls to other countries, is beyond me.

Since no other sources are mentioned, I have to base my conclusion on only these two sources. And fact is that the first source is nothing more than the opinions of some police officers that dared to make some personal estimations, which I wouldn't really call reliable, and certainly isn't any proof.
And the second source has little to do with prostitution in Holland itself. So how Werner ten Kate can come to the conclusion that it's 70% based on these two rapports is a miracle to me. But more importantly, nothing more than guessing based on guessings of other people.

There is no research to prove that 70% of the prostitutes are forced! The mentioned rapports are in one case nearly guesswork of other people, and the second one isn't even about prostitution in Holland itself! Beyond that, both sources did not specifically focus themselves only window prostitution, and also didn't focus themselves on only forced prostitution, but on exploitation as well. Forced prostitution also involves exploitation, but exploitation cases don't involve forced prostitution, which is why we call those exploitation cases, which means the prostitute is NOT forced!

Near guessing, based upon other people their guesswork, based upon personal opinions, using wrong definitions, and not one single prostitute interviewed for the conclusion that '70% would be forced'.
This is not proof of any kind that 70% would be forced. It nearly proves that nobody has any clue how many girls are either being forced or exploited, and people their guesswork heavily vary. There is no scientific research done for this conclusion, and is not based upon facts but opinions!

Dutch version