Perfect reliability isn’t always desirable.
Reliability is the measure of how long a product or system functions properly. Target reliability is expressed as a number between 0 and 1. A target reliability of 1 indicates a goal of perfect reliability; a target reliability of 0 would indicate a goal of all failure. The target reliability of a bridge, spacecraft, pacemaker, or similarly critical system is 1, because failure may result in loss of life. Products that are relatively inexpensive, such as toys or DVD players, are designed with a target reliability of less than 1, because failure is not critical and the cost of achieving perfection would increase cost. Surprisingly, some aircraft parts have a target reliability of less than 1, because of the need to minimize weight. This is mitigated by routine replacements and frequent inspections to identify potential failures.
Failures occur for different reasons over the life of a product or system, with ordinary wear-out failures eventually overtaking start-up (infancy) failures.
