Bias Linearity and Stability

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 Linearity and Stability

 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.

Bias Linearity and Stability

 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

Bias Linearity and Stability

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."

Bias Linearity and Stability

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