text: Simple permutation-based estimation is used to determine how likely a given RP value or better is observed in a random experiment.
  # generate p permutations of k rank lists of length n.
  # calculate the rank products of the n genes in the p permutations.
  # count how many times the rank products of the genes in the permutations are smaller or equal to the observed rank product. Set c to this value.
  # calculate the average expected value for the rank product by: .
  # calculate the percentage of false positives as :  where  is the rank of gene g in a list of all n genes sorted by increasing .