This paper considers an orthogonal frequency division multiplexing system where the Effective Exponential SNR Mapping (EESM) is applied. We first present an approximate model in which the subcarriers grouped into one block are assumed to have the same channel coefficient and, subcarriers in different blocks have independent and identically distributed channel coefficients. Then, we present an algorithm based on a goodness of fit test to estimate the number of subcarriers to be grouped into one block. Building on this approximation model, we provide an approach to compute the statistics of the effective SNR obtained from the EESM mapping. This method is based on convolution technique. The approach presented in the paper is then validated through simulations.