Capability and MSAs are NOT the same thing!

I have found that people often confuse MSA’s and Capability Studies.  Far too often, I hear the question ‘when will we run the capability study on the tester?’   And while I am sure that you few braves souls who read my blog do not fall into this trap, you might know of people who do. Maybe giving them this link will help.

MSA’s are for tests and gages. Capability studies are for the processes being measured. 

Or to state it another way, MSA's give us confidence that we can measure the capability of our process to produce parts to our customers specification.

One can talk about the 'capability' of a tester, but only when the word is being used in its classic sense, i.e. ' the extent of someone's or something's ability.'

Let's review....

Measurement System Analysis 

A measurement system is a collection of procedures, gages and operators that are used to obtain measurements. Measurement systems analysis (MSA) is used to assess the ability of a measurement system using the following statistical metrics;  stability, repeatability (test / re-test variation) and reproducibility (operator variation). 

The most common metric for an MSA is the Gage R&R value. This value is a ratio of the  variation due to the measurement error (repeatability and reproducibility) to the total variation of the system (including both part and measurement variation).

Gage R&R = Variation due to R and R / (Measurement + Part Variation)

Sometimes, one cannot find parts that demostrate part variation to use in the MSA. An example is in electronics with electrical testing (in-circuit tests or functional testing). These systems make hundreds of measurements and it is impractical to attempt to create or find part variation to use in the MSA. 

In these cases we usually run the MSA with ten or so parts off the line. In these cases, the part variation will be very low. Therefore the Gage R&R should be calculated as a percent of the tolerance spec range.

Gage R&R = Variation due to R and R / Tolerance Range

Capability Analysis

From Wikipedia…. “The process capability is a measurable property of a process to the specification, expressed as a process capability index (e.g., Cpk, Ppk, Cp, and/or Pp). 

The output of this measurement is usually illustrated by a histogram and calculations that predict how many parts will be produced out of specification

Two parts of process capability are: 1) measure the variability of the output of a process, and 2) compare that variability with a proposed specification or product tolerance.

Cp (or Pp)  = Spec Range / (6 x total system variation (std dev))

Capability studies assume that the measurement variation is low enough to not be a factor, and that the “total variation of the system’ is effectively due to process and part variation.  

Remember that to do a proper capability study we need a successful MSA,  a stable process, and an large enough sample size to be statistically significant (usually about 90 pieces). Getting a good capability study from prototype builds is difficult due to the (usually) small sample size.