PPM (Portable People Meter)

The PPM is a pager-sized device worn by panelists that passively detects inaudible audio watermarks embedded by participating stations. When the wearer is within earshot of a marked broadcast, the device logs it: no diary-keeping, no self-reporting, no recall bias. At the end of each day the device uploads its logs, giving ratings services genuinely passive measurement of real-world exposure.

PPM replaced paper diaries in major US metros starting in 2007 and fundamentally changed which stations won in measurement. Stations that had depended on diary-keepers over-crediting their favourite formats saw ratings drop; stations with heavy incidental listening (office background, retail environments) saw ratings climb. Programming strategy shifted accordingly.