Digital computers are being used more frequently for process control applications in which the cost of system failure is high. Consideration of the potentially life-threatening risk, resulting from the high degree of functionality being ascribed to the software components of these systems, has stimulated the recommendation of various designs for tolerating software faults. The author discusses four reliability data gathering experiments which were conducted using a small sample of programs for two problems having ultrareliability requirements:n-version programming for fault detection, and repetitive run modeling for failure and fault rate estimation. The experimental results agree with those of M. Nagel and J.A. Skrivan (1982) in that the program error rates suggest an approximate log-linear pattern and the individual faults occurred with significantly different error rates.