• Cume (how many people are listening)
• Time spent listening (how long do they listen)

The best explanation of this is to compare a library to a convenience store. In a convenience store, there is heavy traffic, but the people don't stay very long. In a library, the traffic is lower, but the time spent inside the library is usually longer. See the difference? In order to 

have your message heard in the convenience store 3.29 times or more, you'd have to play your commercial much more often than in the library.

Radio stations certainly are not created equal when it comes to audience size and time spent listening. Accordingly, each one should be bought differently. Contact me for the OES formula to use to determine how many commercials you should run for a proper OES schedule on a given station. This service is free!.

The "do's" of OES:
1)  Run the commercials with guaranteed even distribution across all days and dayparts, Monday through Sunday 6am - 12mid.

2)  Reduce the number of weeks and/or the number of stations being bought…fill one cup before moving on to another. 

The "don'ts" of OES:
1) Do Not change or rotate copy during an OES week! The real issue in advertising is not how much weight or effort has been put behind the project, rather how concentrated that weight or that effort is. It is just as important to concentrate the message you're exposing as it is to concentrate the exposures themselves. 

2) Do not reduce the number of commercials, days or dayparts.

Source: "Radio Advertising's Missing Ingredient: The Optimum Effective Scheduling System", Steve Marx & Pierre Bouvard, 1993.