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A census of customer outages reported to Tandem showing a clear improvement in the reliability of hardware and maintenance has been taken. It indicates that software is now the major source of reported outages (62%), followed by system operations (15%). This is a dramatic shift from the statistics for 1985. Even after discounting systematic underreporting of operations and environmental outages, the conclusion is clear: hardware faults and hardware maintenance are no longer a major source of outages. As the other components of the system become increasingly reliable, software necessarily becomes the dominant cause of outages. Achieving higher availability requires improvement in software quality and software fault tolerance, simpler operations, and tolerance of operational faults.>
Jodi Gray (1990) studied this question.