On posting this I made the prediction that the average assumed by most lesswrong commenters would be above the actual average level of participation.

I hypothesise this is because most LW commenters come from relatively educated or affluent social groups, where political participation is quite high. Whereas there are large portions of the population who do not participate at all in politics (in the US and UK a significant percentage don't even vote in the 4-yearly national elections).

Because of this I would be very sceptical of self reported participation levels, and would agree a quantifiable measure would be better.