Are you so sure about that? From the story:
“Among likely caucus goers, Cain’s support among Iowa conservatives expands to 34.1%, nearly tripling Bachmann’s 12.9% support. Still, Iowa conservatives remain uncertain about the G.O.P. race with nearly one-third saying they are “not sure” who will ultimately win.
The 2012 Iowa Report Poll, a project of Grassfire Nation, is a special survey of Iowa conservatives conducted weekly to help measure where Iowa conservatives stand on the 2012 Republican presidential field.”
Sounds like however they take the poll, they track it week to week and they have their sample down to relevant folks.
Check their website - the poll is not conducted per se, you click on a link. So the population is already self-identifying.