Standard Deviation Index Calculator

What Is the Standard Deviation Index?

The standard deviation index (SDI) measures bias between a laboratory mean and a consensus group mean. It expresses that difference in units of the consensus group standard deviation. Target SDI is 0. Related tools: Standard Deviation Calculator and Relative Standard Deviation Calculator.

SDI Formula

$$SDI = \frac{\mu_{lab} - \mu_{consensus}}{\sigma_{consensus}}$$

A positive SDI means the laboratory mean is above the consensus mean. A negative SDI means it is below. The absolute value shows how many standard deviations apart the means are.

How to Interpret SDI

• 0: No bias.
• 0 to 1: Low bias.
• 1 to 1.25: Acceptable in many lab programs.
• 1.25 to 1.49: Review the method.
• 1.5 to 1.99: Marginal performance.
• 2 or more: Unacceptable bias; corrective action needed.

Example

If the laboratory mean is 9.4, the consensus mean is 8.8, and the consensus standard deviation is 2, then $SDI = (9.4 - 8.8)/2 = 0.3$. The laboratory mean is slightly above the consensus mean but still within one standard deviation.