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SPC Trick 9, Read the Graph - A Wayworld Tutorial
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Here are a few patterns to watch for when you're looking at a Control Chart. With most of these patterns, (or "rule violations" as they are called in real world SPC) the trick is to remember "what are the odds of that happening?" For example, what are the odds of rolling dice and getting a single number five times in a row?
Poor ResolutionPoor Resolution

Change the measurement scale on the Control Chart to widen the graph.

Change in process average (something changed!)8 or more plots above the average:

Indicates a shift in the process Average.

 
7 or more in one direction - the process is going somewhere new!7 or more in a row up or down:

 Indicates a shift in process average.

Alternating 14 times - not a random or "blind" sampling selection14 in a row alternating up and down:

Indicates Measurement Bias, or systematic sampling.

7 in a row the same - very small variation, or poor measurement7 or more in a row with the same value:

Measurement Bias due to pressure/restriction, or excessive rounding off.

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