Considerable discussion about adequate "frequency and duration" of an intervention has been generated within the positive youth development field, associated with issues of program length, intensity, periodicity, and "booster" sessions.
The analysis found that twenty (80%) effective, well-evaluated programs were delivered over a period of nine months or more. A number of these, often those operating in a school domain, applied their interventions during the academic year. In the interventions shorter than nine months, programs ranged from 10 to 25 sessions, averaging about 12 sessions per intervention.
By contrast with the well-evaluated programs, fewer than half (17, or 43%, n=40) of those in the excluded category lasted nine months or more. Once again, those that did so tended to be either those with a reasonable or strong design that failed to prove effects.