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This work is concerned with the effect of grouping data into blocks for transmission in an error detect-retransmit system. A formula is presented giving transmission efficiency in data bits per binit. is a function of the time per bit, block length, nondata bits per block, data bits per word, and average time per block transmission. takes into consideration block length, block starting bit times, mean time to error, turnaround time upon error detection, and correct-block acknowledge time. Using the formula for and the statistics of the bit error rate distribution, an optimum or even variable block length may be chosen. The variable block length system automatically switches when the present efficiency is estimated, with a given confidence, to be nonoptimum.