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DATA COLLECTION, ANALYSIS AND DISPLAY 217<br />

Benefits<br />

It can be difficult to separate out random variation (often called common<br />

cause or non-assignable variation) from real variation caused by changes to<br />

the process. Control charts allow <strong>de</strong>cisions to be ma<strong>de</strong> about processes on<br />

the basis of fact rather than gut feeling.<br />

Example<br />

Figure 1 (on page 218) shows how to select the correct type of control chart<br />

according to type of data and sample size.<br />

Reference<br />

1.S. Oakland and R.F. Followell (1990) Statistical Process Control. London: Butterworth/<br />

Heinemann.

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