Within a US-specific context, I would eschew these comparisons to a notional average and use the following levels of participation:

0 = indifferent to politics and ignorant of current events
1 = attentive to current events, but does not vote
2 = votes in presidential elections, but irregularly otherwise
3 = always votes
4 = always votes and contributes to political causes
5 = always votes, contributes, and engages in political activism during election seasons
6 = always votes, contributes, and engages in political activism both during and between election seasons
7 = runs for public office

I suspect that the average US citizen of voting age is a 2, but I don't have data to back that up, and I am not motivated to research it. I am a 4, so I do indeed think that I am above average.

Those categories could probably be modified pretty easily to match a parliamentary system by leaving out the reference to presidential elections and just having "votes irregularly" and "always votes"

Editing to add -- for mandatory voting jurisdictions, include a caveat that "spoiled ballot = did not vote"