One of the ways to think about how an audience watches TV commercials is to think of viewers as consuming information. It is a back and forth process. The consumer bites off a chunk of information, chews it, and digests it, in order to give it meaning. Then he/she goes back for another bite of information.
What is important for creating memories is not the absolute length of a piece of film, but rather the amount of meaning it contains. To continue our food metaphor, think of it as the nutritional content, not just the calories, in the film. Meaning is, of course, in the eye of the beholder.
One test of the significance of information in a commercial, either on a conscious or unconscious level, is the amount of attention the audience decides to allocate to it. Most pre-testing systems have found that 15-second commercials produce roughly three-fourths the attention-getting power and recall of 30-second commercials.
To understand why, we need to consider the moment-by-moment process by which moving images produce memories.
Only the imagery streaming through an ad that generated the strongest thoughts and feelings of the audience are transformed into the meaningful memories that nourish brands. A 60-second commercial produces, on average, seven peak memories. A 30-second commercial produces, on average, four memories. A 15-second commercial produces three.
So, shorter commercials produce proportionately more short-term memory peaks. This makes sense if you look at “directors cuts” in movies. When time is not a limiting factor, as in the DVD release of a movie, directors will add back in a great deal of information that was left on the cutting room floor when the film was edited down to fit the parameters of a theatrical release. Similarly, when a commercial needs to be edited down to a shorter version, the director will be ruthless in eliminating marginal imagery.
Memory formation, therefore, is not an absolute function of time; rather, it is determined by the rate of meaningful moments produced per unit of time. It is not a matter of how fast you eat your mental food, but how well you chew it.