I live in Wisconsin, and I have never, ever "registered" with a party. The closest I have come is to vote in the Republican primary. This year both the Republican and the Democrat primaries were held on the same day. When I got my ballot I was actually given both party ballots. I filled one out, and then the other was put into a discard box under the watchful eye of the poll watcher. There was no way to assert which party I voted for. How do they then get a weighted scale in Wisconsin.
BTW, by the time we voted here, McCain was the only viable candidate left. I think Huckabee was still technically in the race, but it was a foregone conclusion that McCain had won the primary, so more people in my county voted in the Hillary/Obama (Operation Chaos) race than they did in the Republican primary. I live in one of the most Republican counties in the state. Since the late 1800's we have only voted non Republican once.
When they do their 15k party identification poll each month, there are three choiices, dem, pub or indy. When they do their actual political polling, they weight the samples to these numbers to offset statistical anomalies.
Here’s an example. Their PA numbers are 40% dem, 35% pub and 25% indy. They poll 1100 people but find they were 400 dems, 400 pubs and 300 indys.
They keep the 400 dem responses, throw out 50 pubs and 50 indys to get the 40-35-25 ratio.
BTW - They don’t ask party registration, they ask “affiliation”.