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Precision – Accuracy

Is the accuracy of the measurement you plan to make. This is relevant to the scale and/or detail of your operational definition or description but could also have an influence on the sample size being measured. In any six sigma or ISO-9000 plan, adequate precision is required to monitor process improvements and for the baseline measurements in any benchmarking objectives.

For an example we will set up a test with a histogram plot control chart using the following sample.

Part Number  Revision Part Description
Precision A Accuracy Test

Here is the parameter information.

Parameter Print Loc. Min. Spec. Max. Spec. Nom. Spec.
Accuracy A 1 10 5.5

Using only one decimal point, our chart looks like this.

n x1 x2 x3 x4 x5 X-Bar Range Cp Cpk
1 5.5 5.5 5.5 5.4 5.6 5.5 0.2 21.213 21.213
2 5.4 5.4 5.4 5.5 5.6 5.4 0.2 16.77 16.621
3 5.6 5.6 5.4 5.5 5.6 5.5 0.2 16.77 16.621
4 5.5 5.6 5.4 5.4 5.6 5.5 0.2 15 15
                 
        Totals 21.9 0.8 69.753 69.455
        Avg. 5.5 0.2 17.438 17.364

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Distribution Chart Little Accuracy

Using only two decimal points, our chart looks like this. All we did was modify the original data by a couple of tenths.

n x1 x2 x3 x4 x5 X-Bar Range Cp Cpk
1 5.54 5.56 5.55 5.38 5.58 5.52 0.2 18.576 18.485
2 5.41 5.41 5.39 5.4 5.65 5.45 0.26 13.514 13.369
3 5.58 5.59 5.48 5.55 5.61 5.56 0.13 29.588 29.18
4 5.54 5.65 5.45 5.45 5.65 5.54 0.2 14.985 14.825
                 
        Totals 22.07 0.79 76.663 75.859
        Avg. 5.52 0.2 19.166 18.965

 

Distribution Chart Greater Accuracy

The additional accuracy extends to control charts to include X-Bar and R-Bar.

Also see Quality inspection, DMAIC, (Define, Measure, Analyze, Improve), Control, R & R, (Repeatability and Reproducibility), Tolerance Design, and Variable data.

 

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Last Updated: Saturday, 10-Jun-06 15:50:57 PDT

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