The reliability of a product relates to the probability that the product will fail within a specified time. Reliability is often measured as the mean time between failures or the failure rate per unit of time or other measure of usage. High product reliability is important in such products as airplanes, computers, and copying machines. Stratus Computers. for example. has successfully carved out a niche for itself in the highly competitive computer industry by offering “fault-free” computer systems. The bored May tag repairman with no service calls and Sears Die Hard battery offer additional examples of product reliability.