**Note: **Standard deviation is often abbreviated as SD or s.

The standard deviation measures a test's precision; that is, how close individual measurements are to each other. (The standard deviation does not measure bias, which requires the comparison of your results to a target value such as your peer group.) The standard deviation provides an estimate of how repeatable a test is at specific concentrations. Test repeatability can be consistent (low standard deviation, low imprecision) or inconsistent (high standard deviation, high imprecision).

It is optimum to have repeated measurements of the same specimen in order to have results as close to each other as possible. Good precision is especially important for tests repeated regularly on the same patient in order to track treatment or disease progress. For example, a diabetic patient in a critical care situation may have glucose levels run every two to four hours. In this case, test precision is important since the lack of precision can cause loss of test reliability. If there is a lot of variability in the test performance (high imprecision, high standard deviation), the glucose result at different times may not be true.

Use the following formula to calculate the standard deviation:

A high standard deviation can be attributed to:

- Inherent variability in the test which represents expected error.
- Analytical system malfunction which represents unexpected error the laboratory must investigate and correct.

**Tip: **Levey-Jennings Charts allow you to visually review data points plotted against a ±3SD range. See "Levey-Jennings Chart" for more information.

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