The 1978 report Reliability-centred Maintenance by Nowlan and Heap was based on years of research into all the mechanisms behind failure behaviour and how to deal with it. They discovered that there are six failure patterns. Each failure can be divided into one of these six failure patterns.

Only 11% of all malfunctions are age-related. 89% of all malfunctions are mainly random.

Now, more than 40 years later, this is still the case. This is one of the many pillars on which professional maintenance plans are based.