This formula occured to me one day. The PDI is determined as follows:
In a television series, the ratio of episodes containing core plot developments to filler episodes in a given season. Examples of higher PDI would include most daytime soap operas. Lower PDI examples would include shows like Seinfeld or Friends. On average, shows with higher PDI take a longer time to build addiction, but end up with higher addiction rates.
Depending on the type of show, low PDI can cause boredom and fan revolt. High PDI can result in shows that are difficult to get into, if the resultant history of plot changes are necessary to understand the show.