In this chapter, we describe the method that we will use in most of attacks used in this book. We call it the variance method. For the attacks, we determine conditions that have to be satisfied by inputs and outputs. The conditions appear at random but with a cipher, well chosen differential characteristics can lead to the conditions on the outputs. This is due to the structure of the cipher. Then one has to compare the number of plaintext/ciphertext verifying the conditions. The variance method is a tool that allow to measure efficiently if the difference between the number obtained with a random permutation and the number obtained with a cipher is significant.