A contribution factor is a number for each input variable that is a rough measure of the importance of that variable in predicting the network's output, relative to the other input variables in the same network. The higher the number, the more the variable is contributing to the prediction or classification. You cannot compare contribution factors of different networks. This number is computed in NeuroShell 2 in the Contribution Factors module. This module works for Backpropagation networks only.