There will be several parts to my answer.
As I said, we looked at the study mentioned a while ago and that we put on our website. We tried to determine which groups might be less likely to respond. We determined where they are in order to focus our efforts there. Also, there are all the steps I have mentioned that could be taken to compensate for non-response situations.
Here is another important point. We have never assessed, for most of the variables, the degree of response bias in 2006. So, we do not know the standards which have been established, nor the problems existing in those communities.
Finally, about the reasons why those groups are underrepresented, I have strictly no evidence that they are more likely not to respond. Even though it is commonly held opinion, I have no evidence that they refuse to respond. So, the fact that the survey is voluntary will not really have any effect on the level of representation.