What is Bias Linearity and Stability?

1) Bias:

→ Bias is the difference between the observed average of measurement and the reference value.
→ The reference value, also known as the accepted reference value or master value, is a value that serves as an agreed-upon reference for the measured values.
→ A reference value can be determined by averaging several measurements with a higher level of measuring equipment.

➤Bias Study:

→ Obtain accepted reference value for the part
→ Use layout inspection equipment
→ Measure same part minimum 10 times using gauge under evaluation
→ Calculate the average of readings
→ Bias = Observed Average - Reference value
→ %Bias = [Bias/Tolerance] * 100

➤Possible Causes of Bias:

→ Sensor/Instrument is not properly calibrated
→ Improper use of an instrument
→ Unclear Procedures
→ Human Limitations

2) Linearity:

→ "Linearity is the difference in the bias values through the expected operating range of the gauge.

➤Linearity Study:

→ Select 5 – 8 parts that can be measured at different operating ranges of measurement system
→ Determine reference value for each part using layout inspection
→ Use one appraiser to measure parts
→ Take 10 – 12 repeated readings on each part
→ Calculate part’s Bias
→ Bias = Observed Average – Reference value
→ Plot Bias average against the Reference values
→ Linearity is represented by the slope of the best fit line of these points

3) Stability:

→ "Stability (or drift) is the total variation in the measurements obtained with a measurement system on the same master or part when measuring a single characteristic over an extended time period."

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