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There are three basic types—qualitative techniques, time series analysis and projection,and causal models. The first uses qualitative data (expert opinion, for example) and information about special events of the kind already mentioned, and may or may not take the past into consideration.
The second, on the other hand, focuses entirely on patterns and pattern changes, and thus relies entirely on historical data.
The third uses highly refined and specific information about relationships between system elements, and is powerful enough to take special events formally into account. As with time series analysis and projection techniques, the past is important to causal models.
