The invention relates to a method and to a test device (12) which is used to determine an error rate during the transfer of data (1) in a mobile radio system. Said mobile radio system comprises at least one transfer channel, wherein several data partial flows (2.1, 2.2, 2.6) are transferred. The several data partial flows (2.1, 2.2 . . . 2.6) are produced in a signal generator unit (8). A transport format is determined, in an individual manner, for each data partial flow (2.1, 2.2 . . . 2.6). The data partial flows (2.1, 2.2 . . . 2.6) are sent, respectively, to a number of transfer blocks which belong together (5.1, . . . 7.1). A device which is to be tested captures the transfer blocks (5.1, . . . 7.1) of the data partial flows (2.1, 2.2 . . . 2.6) and evaluates them. According to the accuracy of the evaluation, a positive or negative actuation signal (ACK, NACK) is sent back by the device which is to be tested. An error rate of each data partial flow (2.1, 2.2 . . . 2.6) is determined by the test device (12) from the actuation signals (ACK, NACK) which are captured by the test device (12).