Mark CMG
Creative Mountain Games
Actually, it *is* a caution that negates the collective opinions from being accurately representational. That's the science of statistics. The self-selected group will generally be quantitatively and qualitatively different from the general population.
To say that a group represents the whole is more than just "all opinions that appear in the whole appear in the group". To be properly representational, the opinions of the whole appear in the group with the same frequency, with the same vehemence, and the group should not have opinions that don't appear in the general population.
If you poll a self-selected group, and you poll the general population, the poll results will usually be different. That means the one does not represent the other.
You can even often poll a self-selected group on things that seem entirely unrelated to the selection (for example, asking self-described "fiber artists" about politics) and still not have it match the general population. The biases can run deep and strange.
Actually, it *isn't* a caution that negates the collective opinions from being accurately representational when the sample size is large enough. But you've chosen to only quote part of what I said. Since you ignored a portion of my post where I discussed the sample size being large enough, you've managed to only be correct in regard to what you have posted, not in regard to what I posted in whole. The larger the sample size, the less likely self-selection is a factor in whether the data is qualitatively representational. Furthermore, as to whether it is quantitatively accurate, the larger the sample size, the less likely it is inaccurate as well though any individual opinion is still more difficult to assert as being representational of the same percentage of the whole. In any event, one has to determine if the sample size is large enough to feel the data can be accurate no matter whether you are dealing with a self-selected or a randomly selected sampling of the whole.
The existence of bias can not be predicted as more or less in evidence in self-selected groups then in randomly selected groups. And in point of fact, there is no way to eliminate bias in any type of group. You also cannot say definitively that the biases of any group, self-selected or otherwise, aren't possibly an accurate representation of the biases of the whole. No matter the sample size or the manner of selection, it is paramount to collect large amounts of data and use polling questions that are fashioned to lessen the impact of biases. To offhandedly toss out self-selection in a manner that overstates its importance whenever a discussion of how representional message board opinions can be is a narrow, incomplete view of the science of statistics.