This starts with the definition of Preventive Maintenance. In Reliability Management, Preventive Maintenance is the preventive influence of fault behaviour at fixed intervals. This includes only periodic replacement and periodic revision. State-dependent maintenance, testing and corrective maintenance are not included in Preventive Maintenance.

In the years 1960 – 1978 Nowlan, Heap and Matteson discovered that all types of interference can be divided into 6 categories. These are called failure patterns.

Patterns ABCDEF indicate the probability of failure in time. ABC shows an increasing probability of failure after a certain lifespan. With DEF, the probability does not increase at all after that lifespan. The probability remains constant.

A = 2%
B = 4%
C = 5%
D = 7%
E = 14%
F = 68%