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Defining Phases, Again

The other thing I should have mentioned in my last post is the level at which the phase is defined. We tend to think of Phases as points in time for area probability phases. This is because in a cluster sample, we want to save on travel. Taking a subsample of cases within a cluster doesn't save on travel. So, we tend to use time to find the point at which sampling could occur.

But we could trigger these decisions using some other criteria. A few years ago, I tried to develop a model that detected when there was a change in the cost structure -- that is, when costs go up. The problem was that the model couldn't detect the change until a few days later. Sometimes, it never detected it at all. Still, I like the idea of dynamically detecting the boundary of the phases.

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