I should have said "all were offered encoders" if licensed to the MSA. If they do not install or maintain them, that is a different matter.
In some cases, stations have been sent encoders and they don't install them.
Each station should have gotten a notice from Arbitron when the PPM began testing over 15 years ago, and those that did not encode for the test were offered PPM encoders when the Houston book went "live". And they notifiy stations regularly as well as sending information to the registered address of each licensee.
This is, of course, "you can lead a horse to watter but you can't make him drink". Nielsen and the industry want all stations encoded, as that shows higher PUR making radio seem more important as a mass appeall ad buy.
The minimum for an unsubscribed station to show in the subscriber books is a rating of 0.1, which is about a 1.4 share.