A few follow up questions:

1. Do you expect the opt-in nature of these studies to impact their findings?

2. To compensate for the voluntary nature of the studies, do you think researchers in general will still be incentivized to find data sources that are less respectful of people's privacy and don't require an opt-in to the study?

The Rally participant population is not representative of the U.S. population—these are users who run Firefox (other browsers coming soon), choose to join Rally, and choose to join a study. In research jargon, there's significant sampling bias.

For some studies, that's OK, because the research doesn't depend on a representative sample. For other studies, researchers can approximate U.S. population demographics. When a user joins Rally, they can optionally provide demographic information. Researchers can then use the demographics with reweighting, matching, subsampling, and similar methods to approximate a representative population. Those methods already appear throughout social science; whether they're sufficient also depends on the study.

> 2. To compensate for the voluntary nature of the studies, do you think researchers in general will still be incentivized to find data sources that are less respectful of people's privacy and don't require an opt-in to the study?

Rally is designed to provide a new research capability that didn't exist before. I don't expect a substitution effect like that.

Rally users are all opt-in. How does that impact the design of a Rally study and the conclusions you can draw from it?

There would be no change in terms of research design and the ability to draw scientific conclusions.

edit: also, see https://news.ycombinator.com/item?id=27633212 for details on research design considerations when conducting social science.

So it sounds like a nice shiny cloak for...exactly the kind of data collection nobody actually likes.

Yay for extra steps?

Did the instance you're referencing state it was opt-in then turn out to not be opt-in?