Software doesn’t wear out
• hardware has high failure rate early in its life time
• then the failure drops to the steady level
• the failure rate rises again as time pass
• software has high failure rate early in its life time
• it continues at same rate until obsolescence
• if there are changes, the failure rates will spike
• when a hardware component wears out, it may be replaced, but
there are no software spare parts
• Most software is custom-built, rather than being assembled from
existing components