Beyond accuracy,

we must focus our attention on the variation inherent to the measurement system.

Variation is inherent to the system and we need to understand this variation

due to the measurement system itself.

This opens the door to gauge repeatability and reproducibility.

Gauge repeatability is the variation in instruments obtained

when one operator uses the same gauge for measuring

the identical characteristics of the same products.

The variance affected by the trial to trial measurements is the repeatability.

Gauge Reproducibility is the variation in the average of measurements made

by different operators using the same gauge

when measuring identical characteristics of the same outputs.

The variance affected by the operator or operator measurements is the reproducibility.

The total 100 percent variability found in the system

is equal to the actual part to part process variation,

is what we wish to see and the measurement system

which is preventing us from seeing a clear picture of what's going on.

The measurement variability branch includes both

the gauge variability and the operator variability.

The repeatability variation occurs with the gauge variability

and the reproducibility variation occurs within operator variability.

Reproducibility can be further broken down by the operator to other operators

and operator by part.

Variation due to the operator is called reproducibility.

Variation due to the gauge system is called repeatability.

This is where gauge R&R comes from.

Measurement error sum together.

A set of measurements are taken.

We take several measurements of the same product with the same gauge.

This forms a distribution.

We repeat this for several measurements of several products

to produce multiple distributions.

If we plot the sum of these distributions,

we would see that the total observed variation distribution.

The element we want to uncover is the actual part to part variation.

Measurement variation is often expressed

as a ratio of precision and to tolerance or P over T.

The total variation can be replaced by one-sixth of the tolerance.

So you have the percentage gauge R&R formula is,

100 times the GRR divided by the tolerance divided by six.

Six is used to represent 99.73 percent of the total variation or six standard deviations.

This is where we get six sigma.

The illustration denotes that the standard normal curve

indicating that the area below the curve equals one

but the plus or minus three sigma, or standard deviations from the mean

or six sigma, accounts for the confidence

of capturing 99.73 percent of the total variation.

Let's recap.

The total variation is the sum of the Part to Part Variability, or the actual variability

and the Measurement Variability.

Gauge R&R determines the amount of variation

in the observed measurements due to the Operators and the Equipment.

Calibration insures the equipment mean readings are matched to known standards.

What remains is our actual process variation.

Identifying Bias, Stability, and Linearity all help to improve repeatability measurements.

Various statistical calculations systems such as Minitab

or Microsoft Excel can perform needed distribution analysis.

Statistical process control sometimes referred

to as statistical quality control uses the X bar

and R chart method to identify variability.