
38 Improving reliability and quality for product success
has no infant mortality, which can be precipitated by a short period test-
ing and should consequently always be corrected.
In random failure, the cumulative failure rate is proportional to the life-
time acceptable to customers. Common sense tells us that an item with a 1%
failure rate for 1 year has a 10% failure rate in 10 years. In a practical sense,
this proportionality applies at below about 20% of the cumulative failure
rate, which is the core concept of the exponential distribution function.
*
No
proportionality during use indicates that wear-out failure and infant mor-
tality are mixed ...