How to Decide What Analysis can be Exempted from MSA?

The AIAG's MSA manual requires that five types of analysis shall be done for MSA: bias, repeatability, reproducibility, linearity and stability. An MSA plan should cover all these five types for each measurement system appeared in the control plan as discussed in a previous article, unless it can be justified that certain analysis are not necessary. This article will discuss a couple of examples and explain how one decides what kind of analysis can be exempted from MSA.

Example I:

Caliper I is used for thickness check of materials A with a spec of 5+/-0.01mm. Caliper II is used for thickness check of material B with the spec of B 5+/-1mm. They are not used for any other measurements. In this case, Caliper I is used over a range around 4.99~5.01mm. Over such a small range, the impact of linearity to the measurement result is insignificant. On the other hand, Caliper II is used over a range around 4~6mm and the linearity of the measurement system could be a concern. Therefore study of linearity is not needed for measurement system of Caliper I, but is necessary for measurement system of Caliper II. 

Example II:

An electrical platform scale is used to measure the weight of each product before outgoing. There are two appraisers assigned for this measurement, but for an electrical platform scale, the appraisers should have no impact to the measurement result (all the appraisers do is just to turn on the machine and put the product on the scale. How they do these things do not change the measurement results), and hence in this case study of reproducibility is not necessary.

As a general rule, before deciding what kind of analysis can be exempted from MSA, one must first understand what variation the analysis evaluates. The table below is a summary:

Analysis	Variation Evaluated
Bias	Variation caused by all the factors of the measurement system
Repeatability	Variation caused by equipment itself
Reproducibility	Variation caused by different appraisers
Linearity	Variation caused by measurement range
Stability	Variation caused by time difference

When deciding any of the above analysis can be exempted, one must consider whether the corresponding variation evaluated by this type of analysis has any concern to the measurement results. If not, then the analysis can be exempted from MSA.