This method ensured parity among our two treatment groups. In each stratum, given two possible block lengths of two and four respectively, with a random number generator giving numbers uniformly distributed between 0 and 1, we selected a block length of two for random numbers lower than 0.5 or a block length of four otherwise. Variation of the block sizes prevented the surgeon from guessing what the next treatment will be; it makes it difficult (although not impossible) to break the treatment code.