That's a very good question.
We determined that we would like to try to cover all direct program spending over a four-year period. But you're right, it could have been over a five- or six-year period, which would mean departments would come up once every five or six years.
We did think a four-year cycle made a great deal of sense, but it is really somewhat arbitrary. Once every four years is a reasonable approach for strategic reviews, to put a department through a review once every four years. Once every five or six years may just seem to be too far along. Too many things could change in the department over six years for that to be as useful as a four-year cycle.