kruptof

I am having a bit of a trouble understanding orthogonal array calculation. I am trying to understand the following example given by the lecturer: (not exact words, just what I managed to jot down)

a function that takes 20 inputs of true and false values will have a:

single mode calculation of: 2x20
double mode calculation: (20x(20-1)x2)x4

I think I managed to understand the single mode because there are 2 values and 20 parameters.

I also think I understand some of the double mode. 20x because there are 20 input parameters and the x2 because there are 2 values.

The part i don't understand about the double is 20-1 and x4.

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