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Simple Precision analysis

This procedure is available in the Analyse-it Method Evaluation edition

Simple precision determines the variation of a method observed for a single run (within a device or laboratory).

The requirements of the test are:

  • A method measured on a continuous scale.


Arranging the dataset

Data in existing Excel worksheets can be used and should be arranged in the List dataset layout. The dataset must contain at least one continuous scale variables containing the observations for a run of the method.

When entering new data we recommend using New Dataset to create a new precision dataset.

Determining a simple precision estimate

To start the precision analysis:

  1. Excel 2007:
    Select any cell in the range containing the dataset to analyse, then click Precision on the Analyse-it tab, then click Simple.
  2. Excel 97, 2000, 2002 & 2003:
    Select any cell in the range containing the dataset to analyse, then click Analyse on the Analyse-it toolbar, click Precision then click Simple.

  3. Click Run and select the variable/run to analyse.
  4. Enter Pre-assigned concentration of the analyte observed, if known. If left blank Analyse-it will use the mean of all observations as an estimation of the true concentration.
  5. Click OK to run the test.

The report shows the total number of observations analysed. The Concentration (if the pre-assigned concentration was specified) and the Mean concentration of all replicates is shown. If the pre-assigned concentration is not known the mean is used as an estimate of the true concentration.

The SD, a confidence interval showing the range likely to contain the true precision SD, and CV are shown.

A precision plot (see below) shows the standardized observations from the mean. Observations outside ±4 SDs should be investigated as potential outliers.

Comparing against an imprecision goal specification

Precision can be compared against a performance goal. Allowable precision can be specified in absolute units of the analyte, as a percentage of analyte concentration, or as a combination of the two in which case the larger of the absolute and percentage concentration is used.

To compare precision against a goal:

  1. If the Simple Precision dialog box is not visible click Edit on the Analyse-it tab/toolbar.
  2. Click Compare against and select Imprecision specification.
  3. Enter Allowable imprecision as an absolute value, as a percentage of analyte concentration, or enter both values for a combination.
  4. Click OK.

The precision goal, calculated from the imprecision specification and the concentration level, and a hypothesis test to test if the observed precision is within the goal are shown. If the p-value is significant the observed precision is outside the goal.

Comparing against a TEa and Random Error%

Precision can be compared against a random error% of a total allowable error goal. Total allowable error can be specified in absolute units of the analyte, as a percentage of analyte concentration, or as a combination of the two in which case the larger of the absolute and percentage level is used.

To compare precision against a random error% of total allowable error:

  1. If the Simple Precision dialog box is not visible click Edit on the Report toolbar on the Analyse-it tab/toolbar.
  2. Click Compare against and select TEa, %RE specification
  3. Enter TEa (total allowable error) as an absolute value, as a percentage of analyte concentration, or enter both values for a combination.
  4. Enter % for Random error, the percentage of the TEa to allow precision to vary within.
  5. Click OK.

The precision goal, calculated from the random error specification and the concentration level, and a hypothesis test to test if the observed precision is within the goal are shown. If the p-value is significant the observed precision is outside the goal.

References to further reading

  1. Statistical methods in Laboratory Medicine.
    Strike P.W. ISBN 0-7506-1345-9 1991; TODO (pages).