Abstract: An apparatus and method for measuring errors and event occurrences in a multi-valued data stream by using a dual decision bit error rate tester is disclosed. The Bit error rate tester (BERT) includes a plurality of decision circuits operative to provide a respective bit decision output signal in response to an input signal. The bit decision output signal magnitude information of a signal under test as measured over a sample window period. A comparator circuit is coupled to each of the plurality of decision circuits, and is operative to provide an event occurrence signal in response to the bit decision output signals from each of the plurality of decision circuits. The BERT provides the ability to supply additional information and feedback about the behavior and performance of the targeted device or subsystem being tested and to perform error measurements in non-constrained data (i.e. live data).