AAOM Handbook

Secondary Cause

Any cause of performance loss observed at a Sampling Interval that is not identified as the primary cause of the loss. That is, in the hierarchy of losses these cause are placed below the Primary Cause.

There can be more than one secondary cause for the loss at any sampling interval.

Special cause

A source of variation that is uncharacteristic of the process – that is it is not a ‘normal’ part of the process.

The tests commonly used to differentiate common and special cause variation are; • A po int outside of the ±3σ values. • Seven points above or below the mean. • Seven points moving in the same direction. The probability of each of these events is similar, and is less than 1%.

© McAlear Management Consultants 2004

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