1. A method for estimating the bit error rate in a data transmission system comprising: detecting symbols by a maximum likelihood detector, that provides path metrics of a decoded path and a best path at predetermined symbol positions; calculating absolute path metric differences between the decoded path and the best path (p q c ) at said predetermined symbol positions; and counting events when an absolute path metric difference is equal to one of a set of difference values, thereby obtaining an event count; estimating a bit error rate based on the event count.