Remember that radio did not demand the PPM... the agencies and advertisers kept saying that the results were not timely, not immediate, too old.
So Arbitron created a system agencies liked. Since Arbitron beat The Pulse and Nielsen into oblivion by selling first to agencies, they knew what they were doing when it became time for the PPM. They sold the idea to agencies and radio more or less had to follow ad buyer requests and demands.
The PPM measures real time, and has two "dimensions" which are cume and time spent listening. The diary had three dimensions: it measured cume and TSL, but also measured each listener's memory as generally it was filled in once a day at best.
The biggest differences are in two areas:
First, stations that are secondary to a person are not always remembered by diarykeepers. So they lost lots of diary entries.
Second, in the diary people wrote, for example, at work listening as "9 AM to 5 PM WXXX". In fact, bathroom breaks, coffee breaks, lunch hour, phone calls, meeting or conferences or time away from the work station were not noted; PPM at work listening is in little pieces with lots of interruptions and generally about a third of the diary listening.
The biggest unexpected negative of the PPM is that Persons Using Radio fell by about a third, so agencies began paying much less for spots.
