Arbitron PPM panel is unweighted and has a "statistically accurate" representation of the Seattle market (age, ethnic, zip code etc), plus 300 back-up panelists that can be activated immediately if someone drops out. Average panelist stays 2-3 months before getting tired of carrying the devise around, though some bail after a few weeks. They are booted after 18 months. About 15% of the panel turns over each month. The theory is if the panel is a proper representation of the market it will reflect real changes in the market. The diary was a new 3000 people every 90 days, but the trouble was they could never get a statistically accurate sample so if not enough 18-34 year olds turned in their diaries, the one's that did got double the vote, plus zip codes got under or over represented based on who filled turned in diaries on time and diaries were just best guesses on what someone was listening to, not actual exposure. Caused the wild swings that made advertisers force a new method that relied on a better (not perfect) statistical sample and encoded listening response, not what people thought they heard. Diary made for better posts as stations shot up and down at will. Most of the beef with the PPM is that it measures exposure to radio, but were people really listening?
The PPM has been here 18 months and the entire panel has now shifted over. Still limited change.