It is not our differences that divide us. It is our inability to recognize, accept and celebrate those differences: Andre Lorde
Introduction
We often get so used to sleeping on the same bed for years that when we visit our relatives or on holidays, we often find it difficult to sleep on a different bed. Although the new bed may be more comfortable with our routine bed. Something similar happens with the cup of tea which we use every day to freshen up ourselves.
Objective
Repeatability and Reproducibility are two parts of the same coin. Each has its significance and importance. Once we understand the key difference between them both, the understanding about both enhances our interpretation of the implication of the measurement system and its related error.
Definitions: MSA (Measurement System Analysis) 4th Edition
Repeatability: Variations in the measurements obtained with one measuring instrument when used several times by an appraiser while measuring the identical characteristic on the same part.
Reproducibility: Variation in the average of the measurements made by different appraisers using the same gauge when measuring a characteristic on a part.
Detailed Information
As per MSA Manual 4th Edition and IATF 16949: 2016, both Repeatability and Reproducibility have their specific meaning and intent. Repeatability is also called as Equipment error although it is misleading!
2 key purpose for conducting the study:
- Whether your measurement system variability is small compared with the process variability.
- Whether your measurement system is capable of distinguishing between different parts.
Possible Causes of Poor Repeatability:
- Within Part
- Within Instrument
- Within Standard
- Within Method
- Within Appraiser
- Within Environment
Possible Causes of Poor Reproducibility:
- Between Parts
- Between Instruments
- Between Standards
- Between Methods
- Between Appraisers
- Between Environment
Key difference between Repeatability and Reproducibility:
| S.No. | Repeatability | Reproducibility |
| 1 | Variation in successive trials (short term) under fixed and defined conditions of the measurement | For product and process qualification, error may be appraiser, environment (time), method |
| 2 | Also called Equipment Variation (EV) | Also called Appraiser Variation (AV) |
| 3 | Within Appraiser Variation | Between Appraiser Variation |
| 4 | Inherent Instrument capability or potential | Appraiser Competency |
| 5 | Within System Variation | Between System (condition) Variation |
| 6 | The variation is due to common causes only | The variation is due to operating conditions |
| 7 | Only for manual instruments |
Present Challenges:
- How often the organization is clear about the difference between both?
- How often MSA study is conducted to understand the contribution of measurement error or just to fulfil the IATF requirement?
- How often factual data is recorded to get the actual results?
References:
IATF 16949: 2016
MSA 4th Edition (Measurement System Analysis)- AIAG
Industry Experts
This is the 98th article of this Quality Management series. Every weekend, you will find useful information that will make your Management System journey Productive. Please share it with your colleagues too.
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