# What is Bias Linearity and Stability study in Measurement?

## Bias Linearity and Stability in Measurement:

### ➤ Bias in Measurement:

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.
It is a very important part of a Measurement System Analysis.

#### ➥ 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

### ➤ Linearity in Measurement:

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 the 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
→ Plot Bias average against the Reference values
→ Linearity is represented by the slope of the best fit line of these points

### ➤ Stability in Measurement:

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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