There are several things to remember about forecasting criminal justice events, regardless of whether the forecast is based on simple or complex mathematical models, uses a microcomputer or the most sophisticated main frame computer, or was developed by a high priced consultant. The first point to remember is that the future cannot be predicted with certainty. The rules of probability do not hold in the forecasting of social phenomena such as crime. The further out in time a projection is made, the greater the possibility that the future could vary. Quite simply, a forecast made for tomorrow, one day away, is more reliable than a forecast made for months or years into the future.
Forecasts are guesses about the future based on the past.
As long as the road is straight or curving in wide arcs, the driver can stay on the road by looking backward. However, if a sharp turn occurs or a bridge is out, the driver will crash. So it is in criminal justice forecasting. For example, no one in the 1970's was able to foresee the rapid growth of drugs in the 1980's. Their prognostications were as shaky as are all long range visions.