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Video Link - What is Measurement System Analysis?

What is Measurement System Analysis?

Q: What is Measurement System Analysis (MSA)?

A: It’s a set of techniques that allow us to answer the question:

IF I USE THIS GAGE TO MEASURE,

HOW MUCH CAN I TRUST THE MEASUREMENTS I GET?

MSA is based on Measurement Error

Understanding Measurement Error

Take a single item, and measure it multiple times using the same gage/same technique/same appraiser

Understanding Measurement Error

Record the measurements and draw a histogram

Understanding Measurement Error

The histogram represents a sample from a normal distribution with a Mean, and a Standard Deviation (derived from spread)

Understanding Measurement Error

- Consider the position of the mean – This is how we determine the bias of the system
- Imagine we had measured a reference size – a standard – of a known dimension.
- We could compare the mean of our results to the actual size of the standard.
- The difference is Bias

Bias

Reference Value = 0.500 – This is a known size

Mean of multiple measurements = 0.525

The gage is, on average, measuring about 0.025 too big.

It has a positive bias of 0.025

Bias

The gage is, on average, measuring about 0.025 too small.

It has a negative bias of 0.025

Bias

The gage is, on average, measuring the same size as the standard.

It has a zero bias

Bias

But, we have only dealt with Bias at one size at one point in time

A Gage is used over a range of sizes

And over a period of time

Bias may change depending on these factors:

Linearity

Change in Bias over the operating range of the gage

Stability

Change in Bias over time

Bias

Linearity – Change in Bias over operating range

Bias

Stability – Change in Bias over time

Bias

Stability & Linearity

For standard Gages - How does the calibration system deal with Bias?

Variation

Repeatability & Reproducibility

- Bias is based on the location of the mean of the distribution
- What about the spread of sizes?
- For a perfect gage this spread would be as small as possible
- We need to determine if we can live with the spread (variation) of measurement error from the gage
- We know that when one person uses the gage we get variation – This is the variation inherent in the Gage – REPEATABILITY
- If we have more than one person using the gage, this may introduce more variation - REPRODUCIBILITY

Variation

Repeatability & Reproducibility

Variation

Gage R&R

One way to determine the amount of variation in the Measurement System is through a Gage R&R Study

Take say, 10 parts, 3 appraisers, and measure each part 3 times ( or 15 parts x 4 appraisers x 5 replicates – whatever you want)

Analyze the results using statistical methods e.g. Analysis of Variance (ANOVA) to determine how much variation comes from

Gage R&R

Repeatability

Reproducibility

Variation

Gage R&R

Screenshot from Free Online Gage R&R ANOVA Calculator at www.GageRandR.com

Variation

Gage R&R

Variation

Gage R&R

When we’ve calculated the magnitude of the

Repeatability

Reproducibility

Both together – Gage R&R

We can compare them to:

A Tolerance

A Process Capability

The amount of Total variation in the Gage R&R Study

Anything else that helps answer the question:

How much can I trust my data if I use this Gage ?

- Visit:
- www.GageRandR.com
- For more info about:
- Measurement System Analysis
- Gage R&R
- Free Online Gage R&R Calculator (ANOVA)
- Interactive Process and Gage Simulation
- Other helpful stuff

dave@gagerandr.com