What is Measurement System?
Every decision taken in our organization, society our daily life is based on data. therefore every data is gather using measurement or inspection activity. In industry, lots of data are measure, on that basis they make decision for the product or process. So the collection of operations, procedures, gauges and other equipment, software and personnel used to assign number to characteristics being measured. similarly the complete process is to obtain data by measurement is called the measurement system.
What is Measurement System Analysis (MSA)?
First of all, if our measurement system having an error, then we get the incorrect data. Therefore the decision taken based on wrong data, will result as failure of process or product. So by using statistical study and experiments conducted we analyze the variation present in the result of inspection, measurement & test equipment system. Hence the Measurement System Analysis is defining as the methods of determination of variation in the process due to measurement system. For the reason that the measurement system analysis (MSA) is core element in Production Part Approval Process (PPAP).
The Guideline say that before going for process analysis through Statistical Process Control (SPC), the Measurement system should have the acceptable level of variation. Therefore MSA should be done before SPC.
The source of variation in measurement system is due to the following:
- Man – The appraiser / Operator
- Machine – The measuring device, Gauges, instrument, software etc.
- Material – The object / Part being measure
- Method – Method of test, use of gauge, sample preparation etc.
- Environment – Temperature, Light’s Lux level etc.
So the total variation consist of 5 types
- Process related –
- Part To part Variation
- Within Part Variation
- Measurement system related
- Equipment Variation
- Appraiser variation
- Variation due to interaction
Measurement System Errors
So the measurement system errors can be classified into two categories
1. Accuracy (Location Variation) – It is described as the difference between the measure value and actual / reference value also known as bias.
2. Precision (Width variation) – It is described as the variation in the inspection due to measurement system error also known as measurement variation.
What are the errors to be understand & categories for MSA studies
1. Bias – Difference between measured value & actual value so called bias.
2. Linearity – Difference in Bias over the expected operating range of gage.
3. Stability – Difference in Bias over the extended time period hence called as stability (Time period is days- Stable over time period Uses).
4. Repeatability – It is the variation in obtained while measuring a given characteristic repeatedly on the same part
- With one measuring instrument
- By one appraiser
5. Reproducibility – finally It is the variation in the average of the measurement system made
- By different appraisers
- Using the same measuring instrument
- On identical characteristic of the same part
Types of Measurement System Analysis (MSA) studies / Methods
1. Bias Study
Bias is the deviation of the average observed average of measurement from the true value.
2. Linearity Study
The difference between the bias over the expected operating range of instrument is known as the linearity. “Does your gauge have the same bias over the entire operating range?”
The change in bias over the extended time period is called stability. Therefore the total variation observed with measurement system on the same master part, over an extended time period.
4. Gage Repeatability & Reproduciblity Study
Gauge R & R gives the combine variation of repeatability & reproducibility. This study conducted by two methods,
- Range Method
- Average and Range Method
Click here to learn more about Gage Repeatability & Reproducibility (Gage R&R).
5. Attribute Measurement study
Finally the Attribute characteristics are those characteristics where measurement values is one of a finite number of characteristics or we are not getting any measurement values . the most common example of the attribute measurement system is a go / No go gauge or visual inspection.
Purpose of MSA
- To quantify the variation present in Measurement system
- Ensure the stability of measurement system
- Also initiate appropriate actions to minimize the measurement system variation contributed in total process variation.
Pre-conditions while performing MSA study
- Data should be measure under routine measurement conditions.
- Basic control should be exists, so that it used without required measurement system analysis.
- Equipment should be calibrate
- Adequate discrimination
- Persons are qualified
- Un-necessary causes of variations do not exist etc.