12. MSA – Introduction
Measurement System Analysis (MSA) is a way to evaluate your current measurement system’s effectiveness and suitability, the major variance indicator which impact the measurement system are part to part variation and the variation within the measurement system itself.
In measurement system, accuracy (variance coming from measuring value and true value) and precision (variance coming from measuring repeatedly with the same gauge for same part) are two critical indications of the measurement system’s effectiveness. The following graph indicate the possible situation with the measurement system.
In general, 5 elements inside measurement system shall be considered:
- Linearity
- Constant relationship between output variable and input variable as changes occur.
- Bias
- Systematic error of measurement, is the difference between observed average of measurements and reference value.
- Repeatability
- Variance coming from measuring instrument equipment, or performed via the same operator with the same gauge to measure repeatedly.
- Reproducibility
- Systematic variance coming from different operators using the same gauge to measure.
- Stability
- Ability of measuring system to maintain constant metrological characteristics over time.
- Linearity
But in order to analyze the critical indicators of system, Gage R&R (Repeatability & Reproducibility) is commonly used in the industry for the following purposes:
- Comparing process variance with measurement system variance.
- Comparing product characteristic’s specification limit with measurement system variance %.
- Appraiser variance’s contribution with respect to measurement system variance.
- Can measurement system identify individual measured parts?
Prior to evaluating the measurement system, the process map below will give a general layout of respective steps before giving the final evaluation regarding your measurement system.
There are 4 types of Gage R&R which can be referred for measurement system analysis. And the following diagram is the indication of which type of analysis is applied based on measurement system’s nature.
And the table below indicates the general acceptance standard for individual Gage R&R type.
2 Comments
Join the discussion and tell us your opinion.
The different types 1 – 3 and attributes, is this driven by a Std. such as AIAG? We got a customer asking for only a Type 1 and Type 3 only reports.
Hello Charles, it is referred from the ASQ definition alongside some other countries’ quality standard definition for measurement variances. The measurement system’s nature will dictate what type you will be applied for.
Type 1 is to generally make sure your gauges are within compliance when verifying with your reference standard. And I assume type 3 is mostly due to only one operator or one gauge is available to obtain your data, that is why your customer wants to see only the repeatability result. Otherwise it would be Type II due to the fact different measuring instrument along with different operators would cause cross variation. I hope this clarifies some question from your side.