# Intro to Measurement System Analysis (MSA) of Continuous Data – Part 2: Bias

#### Miner

##### Forum Moderator
Staff member
This is the second in a series of articles about MSA. The focus of this article will be on measurement bias, sometimes referred to as accuracy.

Bias is the difference between the actual value of a part and the average measured value of that part. In other words, a measurement device that has bias will consistently over or under state the true value of the part.

In most cases, a separate study of measurement bias is not performed if the measurement device has been calibrated. The reason for this is simple. Calibration is intended to detect and correct any measurement bias found. As I stated in Part 1, calibration and measurement uncertainty are outside of the scope of this series and is better left to experts in those fields. However, I will state that all calibration programs are not created equal. Some less equal calibration programs may take a single measurement of a standard and then make a determination on whether there is measureable bias in the gage. This overlooks the fact that taking a second or third measurement could provide different results than found in the first measurement. There are also other less obvious sources of bias from which a calibration system, no matter how well designed and implemented, will not protect you.

I will go through a few examples of bias that you could encounter:
• Measurement device bias – As we discussed in Part 1, all measurements vary to some extent. if the device has sufficient resolution to see it. The failure mode of a weak calibration system is to base the calibration on a single measurement. The solution is to take multiple measurements of the standard and compare the average of these measurements to the standard before making a determination of the magnitude of the bias and making an adjustment. Even better, a 1-sample t-Test may be used to determine the statistical significance of the bias provided the required sample size is established in advance using the maximum allowable bias and desired alpha and beta risks.
• Temperature bias – Many products will change size with changes in temperature. The magnitude of this change in size may or may not add significant bias depending on the materials involved. What may be less commonly known is that the measurement device will also change size with temperature. How often does the appraiser carry the measurement device in a pocket or in their hand warming it up to body temperature? Temperature not only affects mechanical dimensions, but also electrical. Resistance changes with temperature affecting many electrical measurements. An extremely important aspect of calibration performed by internal lab is normalizing both the standard and the gage at standard temperatures before calibration.
• Humidity bias – Certain materials will swell or shrink with changes in moisture content. Critical measurements should be made at standard humidity conditions after a lengthy normalization time.
• Pressure bias – Materials that are compressible such as rubber or foam are notoriously difficult to measure due to the deformation of the part under pressure. But did you realize that the steel shaft diameter that you are measuring may also be understated depending on whether you used the ratchet thimble on the micrometer or not?
• Cosine error bias – Not just for CMMs! Test indicators, less commonly used these days, are also susceptible to cosine error. Did you realize that the ball on the tip of a CMM probe can introduce potential bias? All touches made with the tip must be made perpendicular to the surface of the part. When this is not done the diameter of the sphere will introduce what is called cosine error. The larger the sphere used, the larger the cosine error.
• Measurement procedure bias – Your measurement procedure can also introduce bias. How do you measure the diameter of a shaft (randomly, max, min, average of max-min, using CMM)? What about the location on the shaft (middle, end, multiple locations)? The effect of this bias depends on the application of the part. Does the shaft need to slip into a hole? The average diameter reported by a CMM will understate the effective diameter of the shaft. It may measure in specification and not fit into a ring gage.
The next article will be:
Intro to Measurement System Analysis (MSA) of Continuous Data – Part 3: Linearity

Last edited by a moderator:

Staff member
Thank you!

G

#### Guest

hello sir
This is suresh & just started to learn about the studies in MSA,If you have any excel sheets showing step by step how to calculate only bias including formulas & tables involved in it & what should be the acceptance criteria. It will be great for me if u support pls

#### Miner

##### Forum Moderator
Staff member
suresh01;bt530 said:
hello sir
This is suresh & just started to learn about the studies in MSA,If you have any excel sheets showing step by step how to calculate only bias including formulas & tables involved in it & what should be the acceptance criteria. It will be great for me if u support pls
Look for my attachment MSA 3rd Ed - ANOVA.xls file. There is a tab for the different MSA studies beyond R&R. Bias is included.

#### Marc

##### Hunkered Down for the Duration with a Mask on...
Staff member
Miner, your help and participation here is very much appreciated!

Intro to Measurement System Analysis (MSA) of Continuous Data – Part 5b: R&R Imported Legacy Blogs 14
Intro to Measurement System Analysis (MSA) of Continuous Data – Part 5a: R&R Imported Legacy Blogs 5
Intro to Measurement System Analysis (MSA) of Continuous Data – Part 4: Stability Imported Legacy Blogs 2
Intro to Measurement System Analysis (MSA) of Continuous Data – Part 3: Linearity Imported Legacy Blogs 15
Intro to Measurement System Analysis (MSA) of Continuous Data – Part 1: What and Why? Imported Legacy Blogs 22
Intro to MSA of Continuous Data - Part 10: Creating gauge families Imported Legacy Blogs 10
Intro to MSA of Continuous Data - Part 9: %Tolerance for One-sided Specs Imported Legacy Blogs 3
Intro to MSA of Continuous Data – Part 8: Comparison of Two Gages Imported Legacy Blogs 3
Intro to MSA of Continuous Data – Part 7: R&R using Wheeler’s Honest Gage Study Imported Legacy Blogs 17
Intro to MSA of Continuous Data – Part 6: R&R for Non-Replicable Measurements Imported Legacy Blogs 18
T Intro & ISO 9001:2006 Question - Is ISO 9001:2006 Released? ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 1
E French Intro to Quality - Introducing ISO 9001 to New Employees Book, Video, Blog and Web Site Reviews and Recommendations 1
QMS monitoring, measurement, analysis and evaluation requirement - Template ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 3
Physical Part Measurement General Measurement Device and Calibration Topics 6
Measurement Uncertainty in Optical Microscopy Measurement Uncertainty (MU) 1
ISO 6508 and portable hardness measurement instruments General Measurement Device and Calibration Topics 0
How to Establish the Calibration & Measurement Capability (CMC)? ISO 17025 related Discussions 1
Measurement of residual magnetism General Measurement Device and Calibration Topics 2
Surgical angle measurement guide device with an application software Medical Device and FDA Regulations and Standards News 1
Disabling measurement data during fault conditions IEC 60601 - Medical Electrical Equipment Safety Standards Series 5
Most versatile measurement device General Measurement Device and Calibration Topics 2
Surface scratch measurement General Measurement Device and Calibration Topics 1
Drawing tolerance vs. Measurement device General Measurement Device and Calibration Topics 7
MSA on automated measurement system - Multiple Step Vision System Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 3
Thermo-Hygrometer - Measurement of 10DegC @10%RH ISO 17025 related Discussions 4
GDT Flatness measurement question Inspection, Prints (Drawings), Testing, Sampling and Related Topics 10
Software Validation for Measurement Tools used in Process Validation ISO 13485:2016 - Medical Device Quality Management Systems 2
Best Measurement Systems Demos in California? General Measurement Device and Calibration Topics 1
Measurement Error - How to determine what is acceptable? Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 3
Process Mapping - Inputs/Outputs/Detail Activities/Control points/Measurement? Process Maps, Process Mapping and Turtle Diagrams 4
IEC 60601-2-10 - Accuracy of Pulse Parameters - Required Measurement Uncertainty IEC 60601 - Medical Electrical Equipment Safety Standards Series 3
Program or application for standard time measurement in industry from video surveillance? Human Factors and Ergonomics in Engineering 1
Defining frequency of measurement tools callibration Calibration and Metrology Software and Hardware 3
How to “validate/qualify” a new measurement equipment? Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 4
Verification and Validation of Measurement Systems Misc. Quality Assurance and Business Systems Related Topics 0
How do you add accuracies for tools with different measurement devices? General Measurement Device and Calibration Topics 1
Viable count Measurement ISO Standard Other Medical Device Related Standards 1
Gauge R&R on a torque measurement Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 11
Air Quality Measurement Hardware and Software General Measurement Device and Calibration Topics 11
Dimension Measurement Tool Recommendation General Measurement Device and Calibration Topics 21
Does pitch/increment/resolution of a ruled scale apply to measurement uncertainty as line item? Measurement Uncertainty (MU) 10
Plane measurement with a 3D touch machine like a CMM. Calibration and Metrology Software and Hardware 3
Diameters measurement with 3D touch machine like a CMM Calibration and Metrology Software and Hardware 4
Uncertainty in measurement larger than tolerance Measurement Uncertainty (MU) 2
Measurement Uncertainties Budget for Thermometer Measurement Uncertainty (MU) 6
Performance Measurement ISO 9001: 2015 ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 2
Monitoring performance - How do I determine performance measurement basis within my organization? Misc. Quality Assurance and Business Systems Related Topics 4
What information to put on measurement Dimensional Results APQP and PPAP 7
O Ring capability and measurement - What is the automotive 'norm' for capability studies on O Rings? General Measurement Device and Calibration Topics 3
Electrical Metrology (Source/Generation and Measurement) General Measurement Device and Calibration Topics 1