The concept is very simple:
NNs are good interpolators.
NNs are NOT GOOD extrapolators.
Regardless of the physical source of the data:
1. The input and output must be correlated. This does not necessarily mean that there is a causal relationship. Both may be the result of the sanother source, known or uknown, for which there are no direct measurements.
2. If the statistical characteristics of the nontraining data are not similar to those of the training data, the net should not be expected to perform well.
If new data does not fulfill this criterion, a new net with representative training data should be designed.
Hope this helps.
Greg
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