For studies in which people have to actively involve themselves and consent to participate, I believe that there is always going to be some sampling bias. At best we can make it really really small, at worst, we should state clearly what we believe are those biases in our population.
At worst, we will have a better understanding of what goes into the results.
Also, for some studies, the sampled population might, by necessity, be a subset of the population.
Thanks so much! I didn’t know about them, it’s a good datapoint. Do you happen to know if they are active?