This is often a very time consuming component of the project and can slow the team’s quick progression through the process.

Continue to focus on low hanging fruit that may be momentum sustainers and work vigorously through the MSA process. Most of this can be done by the GB/BB outside of the team meetings and results shared with them when complete.

Accuracy / Bias

For best accuracy of the data:

1) Accept all data as it is collected. Assigning special cause and scrutinizing the data can come later.

2) Record the data at the time it occurs.

3) Avoid rounding off the data, record it as it is.

4) On the data collection plan, record as many details around the data such as the exact source, machine, operator, conditions, collector’s name, material, gage, and time. Record legibly and carefully.

The data should be screened for misplaced decimal points, duplicate data entries by mistake or improper recording procedure, missing date points if frequency is important, and other obvious non-representative data.

5) Verify the gage is accurate. If using a weigh scale, verify it with a known and calibrated weight. Use gage blocks for calipers or micrometers. Use hardness blocks to verify hardness testers.

Resolution / Discrimination

Adhere to the 10-bucket rule. If your measurement system requires measurements to the hundredths (x.xx), then divide that by 10. Collect and record the data to the nearest thousandths (x.xxx). The measurement system shall be sensitive to change and capable of detecting change.

The lack of resolution will not allow a measurement system detect change. If you are measuring the downtime and using measurement to the nearest hour and most downtime is less than an hour then most of the reading will either be a 0 (for 0 hours) or a 1 (for 1 hour).

However, using a stop watch and recording data to the nearest minute will provide 60x more resolution and allow better distribution of data points, more variety of data, with fewer repeat measurements. You could have 60 different readings. Actually recording the nearest 6 minutes would satisfy the 10-bucket rule, but it is a guide to help ensure resolution in the measurement system.

This part of the MSA is usually the easiest to fix such as finding a micrometer, caliper, hardness tester that can capably read to the next nearest decimal.

TROUBLESHOOTING:

Try acquiring a larger samples size, with the idea that some of these may create new observations or measurements.

Measure to as much resolution as possible and practical.

Linearity

Stability

SPC Charts use a variety of tests to determine stability. Many software programs will have these as options to include when analyzing data and will even indicate the point(s) and test that each failed.

Some of the corrective measures once again include Standard Operating Procedures. Each appraiser should measure the same way every time over a long period of time and each appraiser should measure the same way as all the others. Recall that special causes can also occur with the process control limits and these must be given corrective action before proceeding to validate the measurement system.

Gage R&R

Variable Gage R&R

When performing the replicated appraisals it is critical that the measurement are randomized so that no patterns or predictability can be entered in by the appraiser. This bias will mislead the team and create a useless Gage R&R.

For example, an appraiser may remember the 7th part that was measured was borderline and made a decision to give it one measurement. He/she may have spend a lot of time of that part and if the 2nd round of measurements are not randomized, that person will remember the measurement (appraisal) they gave it on the first round.

So, move the parts around each repeat set of measurements. However, the parts must be indentified so the person entering the data into the statistical software enters the reading under the correct part.

Four Criteria in Variable Gage R&R

Ideally, all four categories should be in the GREEN zone. Examining the visual aids below shows commonly used judgement criteria for each category.

2) % Tolerance

This study shows the measurement error as a percent of tolerance in short period of time. It oncludes both repeatability and reproducibility, can not be separated.

5.15 Study Variation = 99% (constant)

The TOP TABLE at the top is a part of the d2 distribution. This value is a constant that is found by looking at the column with 3 appraisers and going across with the row with 6 parts. In this example the d2 value is 1.73.

The LOWER TABLE shows that actual measurements that each of the appraisers cam up with using their variable gage. The range of the three measurements for each part is shown on the right. Then the average range is shown (=0.69) and this is carried on to the Gage Error formula.

The process tolerance is the difference in the specification limits. For example, if the USL is 27 and the LSL is 2, then the tolerance is 25.

With the tolerance being 25, then:

Ability of one appraiser to get the same result and another appraiser or the ability of all appraisers to get the same results AMONG each other.

To optimize reproducibility in ATTRIBUTE Gage R&R:

1) Create visual aids, templates, definitions, or other specific criteria for each to meet a certain rating, value, or appraisal. Pictures of good, bad, in the middle, and colors, will help each appraiser standardize their response, improving the reproducibility.

Note: If these corrective actions are needed to pass the Gage R&R, it should be instituted as a formal work instruction and everyone involved throughout the company or plant should adhere to same instructions.

To optimize reproducibility in VARIABLE Gage R&R:

1) Create a Standard Operating Procedure with visual aids and definitions. When using humanly subjective "touch" devices such as micrometers and calipers it is important that all appraisers "squeeze" the same amount. Too little or much pressure at higher levels of resolution can be enough to alter the Gage R&R.

2) Visual aids also help. Even when using an optical comparator to get a higher resolute data point there is subjectivity where to place the template or the starting and end point(s) on the shadow. Pictures of acceptable and non-acceptable will help reduce this variation.

REPEATABILITY

This describes the ability for an appraiser to repeat his/her measurements each time when analyzing the same part, unit, etc. In destructive testing (such as tensile testing) these reading will not be possible and some statistical software programs have options to select for destructive testing.

The goal is to have an appraiser repeat unit readings at least three times. The person administering the test should randomize the sequence each time to prevent and patterns and bias (the appraiser may remember or try to remember what a measurement was and tend to alter real measurements to get the Gage R&R to pass). It is important for the administrator to record carefully to ensure readings correlate the correct part/unit each time.

Avoid writing down measurements and then typing them into a statistical program. The fewer times measurements are recorded and copied the lower the risk for human error to add even more variation and possibly fail (or pass) the Gage R&R when it shouldn't have.

Precision is the ability to have the same repetitive result (or appraisal in this case). Visually, it means that all your shots of an arrow are very close to one another. It does not mean that they are near the bulls eye. In other words, it does not mean that your shots are accurate.

If your shots are accurate and precise, then they are tight circle centered around the target. It is also possible to be somewhat accurate without being precise. You may have several shots all around the bulls eys (target) but they may be scattered around it. If you take a look at the group the center (mean) may be the bulls eye but the shots are not in control or precise. In others words, there is a lot of unpredictability or variation.