The present invention relates to a method and arrangement for providing a reliability information on information bits (bk) transmitted in a communications network, said bits constituting a transmitted vector (s) comprising elements selected by a transmitter from finite alphabet (S), wherein said transmitted vector is detected from an observed vector received symbols (y) providing a log-likelihood (L). Thus, the approximation is done by defining a number of bit terms having constant size and selecting a subset of bits and marginalizing over a selected part of said subset of bits.