Aspergers Survey Re-results

That is exactly what Psychohistorian expected.

Also, we don't know that there was "no bias at all" for taking the survey, just that the net change in bias between the first group and the second group was relatively small. I expected there to be a big bias in who made it through the first filter (reading the original article and finding out about the poll in its last paragraph) and multiple additional biases which would partially cancel out, and predicted that the net effect of these additional biases would be for the second group's AQ to be a bit lower than the first group's (but still substantially higher than the true population AQ). For instance, only some of the nonresponders from the first round actually saw your request for them to take the second-round survey, and they might tend to be high-AQ because they needed to look at your second Aspergers post to see the request.

I made this prediction about the AQ scores with low confidence, since it's hard to guess the relative sizes of all of these potential biases, or even to identify every relevant bias. For instance, the data are showing that there were slightly more people who have been diagnosed with Aspergers in the second round than the first, and this could reflect a genuine difference (rather than random variation) caused by another bias: people who have been clinically tested for Aspergers might have been more likely to read the post but less likely to go on to take the survey, since they had less to learn from it.

My main point in this discussion is that, for future surveys, it's better to try to avoid selection effects in the first place than to try to account for them after the fact, since they can introduce a lot of uncertainty which is hard to get rid of.