Comment by starbugs

> Opt-in makes the data useless - not just in terms of the huge drop in quantity but because of the fact it introduces a huge bias in the data selected - the people that would opt-in are probably not a good sample of "typical users".

Why? I don't think that's obvious. It may also be related to the way the opt-in is presented. In general, I would expect this to be a workable solution. Even if the opt-in group deviates from the "typical user", it's the best data you can get in an honest and ethically sound way. This should certainly be better than no data at all?

For any website/app that presents an opt-in cookie consent banner this is implicitly already the case.