In a highly absorbing listening space with plush carpets and padded chairs, high frequencies are more readily attenuated than low frequencies. Some listeners may use a radio broadcast as background and not care that they are only hearing the low frequencies. Depending on the loudspeaker in the radio, high frequencies may project in a highly directional narrow beam while the low frequencies are omnidirectional. High-frequency content may also be blocked and absorbed if the listener has attached the portable PPM monitor to his or her clothing in a way that it faces the upholstery of a plush chair or if the listener has placed the device in a purse or backpack.

Given the wide range of listening environments, ranging from always decoding to never decoding, a radio station can influence the PPM performance in the intermediate cases. Marginal cases will decode correctly if the watermarking energy is made stronger, and this factor is influenced by the properties of the audio source. Even though radio stations can only influence encoding, they should keep in mind the full system of encode and decode processes.

Hidden Assumption -- The Influence of Randomness

Statistical measures of reliability have yet another hidden assumption: randomness.

Even if the PPM system were only 90 percent reliable, the system would be acceptable if the errors were distributed uniformly across all programs and listeners. Errors would average out. However, a system that was an impressive 99 percent reliable over all announcers, program types, and listening devices might have the failures concentrated in a particular intersection of programming type and listener subculture. The fallout from such a failure might be further amplified by an inadequate audience sample size.

Statistical assertions about accuracy almost always assume that bell-curve models represent the real world. But very few real-world situations actually fit the bell-curve model. If the errors are not random, there is a problem. For an in-depth treatment of this kind of statistical failure, see Nassim Taleb's book The Black Swan: The Impact of the Highly Improbable.

Avoiding "Gaming" of the System

In the absence of empirical data about the frequency and severity of failure scenarios in the real world, we can only speculate about their existence. Nevertheless, I conclude that there will be some cases where the PPM system fails simply because the program content and listener environment do not match the assumptions made by the designers of the PPM system.

A radio station has no direct way of modifying how portable decoders behave, nor should it. Gaming the system at the listening end would be no different from stuffing the ballot box or falsifying paper diaries. It is in the industry's best interest that a firewall be maintained between stations and individual participants in the audience measuring process.