From Owen: An orthogonal array A is a matrix of n rows, k
columns with every element being one of q symbols
0,...,q-1. The array has strength t if, in every n by t
submatrix, the q^t possible distinct rows, all appear
the same number of times. This number is the index
of the array, commonly denoted lambda. Clearly,
lambda*q^t=n. The notation for such an array is OA( n, k, q, t ).