Understanding Variation: A Guide to Attribute MSA Studies

Attribute MSA Study:

Attribute MSA is a tool used to evaluate the measurement system when attribute (qualitative) measurement is to be done. In attribute MSA study we check the measurement error is within an acceptance level before doing SPC study.

Attribute and Variable in MSA:
- Attribute Data (Qualitative):
- Variable Data (Quantitative):
Types of MSA:
Steps to perform Attribute MSA study:

Attribute and Variable in MSA:

Attribute Data (Qualitative):

Yes / No
OK / NG
Good / Bad
Go / No Go
Pass/ Fail
On-Time / Late
Category like M/C 1, M/C 2, M/C 3

Variable Data (Quantitative):

Dimensions
Cycle Time
Pressure
Speed

Types of MSA:

1. Attribute MSA

2. Variable MSA

Bias
Linearity
Stability
GR & R

Types of Variations in attribute MSA study:

Variations within an appraiser’s repeated measurement
Variations between different appraisers
Variation between the appraiser’s measurement value and actual value (Standard value)

Why is an MSA study required?

MSA study is required for the following conditions:

New manufacturing process
New manufacturing product (NPD)
New equipment
Any customer complaint
Any internal quality issue

The major source of variation:

Part-to-part variation:

This is the variation between parts (process variation). Approx more than 95% variation is between parts.

Measurement System Variation:

Variation due to inspector (Reproducibility). It is also called Appraiser variation.
Variation due to gauge (Repeatability). It is also called Equipment variation.

Types of error in MSA:

Type 1 error:

The good part is sometimes called the bad
It is also called False Alarm
There is Producer Risk in this condition

Type 2 error:

The bad part is sometimes called the good
It is also called Miss rate
There is Consumer Risk in this condition

Steps to perform Attribute MSA study:

Select the gauge to be studied for attribute MSA study
15 parts are to be selected so that some parts are at the borderline from the regular production. (More sample parts better will be the result). Take some NG parts also for study. For example, as in the format below, we have taken 11 parts OK and 4 parts NG.
Select 3 different appraisers/inspectors who are performing the particular MSA study.
Ask the inspector to check and give a decision for each sample 3 times in random order, and the observer will record the result. ( We can study 5 times also as given in format)
Enter the values in the attribute MSA study format. For OK parts write as 0 and for NG parts write as 1
Analyze the result: 1) Kappa value 2) Miss rate & False alarm
Take decisions for the acceptability of the measurement system.

Attribute MSA Study Format:

MSA-Attribute Download

Attribute MSA acceptance criteria:

The effectiveness of each appraiser should be more than 90%
Miss Rate of each appraiser should be less than 2%
False Rate of each appraiser should be less than 5%

Kappa Value:

Kappa is an indicator of interrater agreement. It means the ratio of the proportion of agreement divided by the maximum number of times they could agree.

Po = The sum of the observed opportunities in the diagonal cell.

Pe = The sum of the expected proportion in the diagonal cell.

A Kappa co-efficient of 85% may be interpreted as 85% agreement.

Attribute MSA study questions:

Questions:

Q 1. After which core tool SPC is done?

Ans. a) APQP b) PPAP c) FMEA d) MSA

Q 2. What are the acceptance criteria for a False alarm?

Ans. a) 3% b) 2% c) 5% d) 10%

Q 3. What are the acceptance criteria for Miss rate?

Ans. a) 3% b) 2% c) 5% d) 10%

Q 4. Which is higher in Part variation and measurement system variation?

Ans. a) Measurement system variation b) Part variation

Q 5. Which type of MSA is done for snap gauge?

Ans. a) Attribute MSA b) Variable MSA c) Both

Q 6. Which is not the attribute data below?

Ans. a) Pass b) OK c) 2.5 mm d) No Go

Q 7. Which are the variables data in below?

Ans. a) Dimension b) Speed c) Cycle Time d) All

Q 8. In the MSA format given 0 denotes for which part?

Ans. a) OK part b) NG parts

Q 9. In the MSA format given 1 denotes for which part?