a method is provided for compressing relatively time invariant binary data , such as speech data in a telephone answering device , using statistical analysis of changes in the data . an original record organized into multiple frames of multiple bits each is used to construct an xored record of the same number of frames and bits . the xored record has a base frame with the same bit value pattern as a corresponding base frame of the original record , and remaining frames with bit values given by the outputs of an exclusive - or operation applied to the bit values of corresponding and prior frames of the original record . the bit positions of the xored record frame set are analyzed and reordered , according to their bit value change activity and used to construct an output record . the output record may have a base frame with the same bit value pattern as the corresponding reordered xored record base frame . other output record frames are established using a compression scheme wherein at least low bit value change subframes of the reordered xored record frames are compressed by replacing them with shorter bit patterns having a format comprising a first part representing the number of bit changes occurring in the subframe and a second part identifying the location or locations , if any , of those changes . the foregoing procedure is reversed to restore the original record from the output record .