Estimating the bias of a measurement system

Estimate the trueness of a measurement system or measurement procedure.

  1. Select a cell in the dataset.
  2. On the Analyse-it ribbon tab, in the Statistical Analyses group, click Trueness.
    The analysis task pane opens.
  3. In the Y drop-down list, select the measured quantity variable.
  4. If there are multiple levels of measurement, in the By drop-down list, select the level variable, and then:
    • If the values are identifiers, select the Identifier check box, and then in the Assigned values grid, under the Value column alongside each level, type the value.
    • If the values are relative dilutions made by mixing high and low pools, select the Relative values check box, and then in the Assigned values grid, under the Value column for the first and last level, type the value (intermediate values are automatically calculated using relative values).
    • If the values are known/expected/assigned values, select the Known values check box.
  5. If the reference materials package insert includes an estimate of the uncertainty of the assigned values, select the Uncertainty in assigned values check box, and then in the Assigned values grid, under the SE column for each level, type the value:
    • If the estimate is expressed as standard uncertainty or combined standard uncertainty, use the value.
    • If the estimate is expressed as a 95% confidence interval (CI) use the value CI/2, or if the confidence interval is expressed as upper and lower limits use the value (CI upper limit- CI lower limit)/2.
    • If the estimate is expressed as expanded uncertainty (U) with a coverage factor (% or k) use the value U/k. If coverage is 95%, use the value U/2; if coverage is 99%, use the value U/3.
  6. Optional: To compare the bias against performance requirements:
    • If the allowable bias is a constant or proportional value across the measuring interval, select Across measuring interval, and then in the Absolute edit box, type the bias in measurement units, and/or in the Relative edit box, type the bias as a percentage (suffix with % symbol).
      Note: The allowable bias is the greater of the absolute bias and the relative bias for each level. Therefore, with a absolute bias of 5mg/dL and a relative bias of 10%, the allowable bias will be set at 5mg/dL for all values 0 mg/dL up to 50mg/dL and then at 10% of assigned value for values above 50mg/dL.
    • If the allowable bias varies for each level, select Each level and then in the Allowable bias grid, under the Absolute / Relative column alongside each level, type the bias in measurement units, or the bias as a percentage (suffix with % symbol).
    Note: To compare if the value of the reference material is equal to the assigned value, see Testing against an assigned value.
  7. Click Calculate.