Standard Deviation Index Calculator
Enter an observed value, target mean, and target standard deviation to evaluate how many SDs the observation deviates from the target.
Standard deviation index (SDI)
-1.0000
Within ±1 SD — good agreement.
How to Use This Calculator
- Enter the observed test value from your laboratory or process.
- Provide the target mean and the target standard deviation supplied by quality controls.
- Review the SDI to determine how far the observation deviates in standard deviation units.
- Interpret SDI magnitude to decide whether corrective actions are needed.
Formula
SDI = (Observed − Target mean) / Target standard deviation
SDI expresses deviations relative to a reference SD, often used in clinical laboratories and quality control programs.
Full Description
Standard deviation index quantifies how far an observed result is from a target mean in units of the target standard deviation. Values within ±1 typically indicate good agreement; values beyond ±2 warrant investigation to ensure process reliability.
Frequently Asked Questions
Can SDI be used for proficiency testing?
Yes. Laboratories often report SDI to monitor performance against peer or target values.
What SDI indicates failure?
Many programs consider |SDI| > 2 unacceptable. Check specific quality guidelines for thresholds.
Why do I need the target standard deviation?
SDI scales deviations relative to expected variability, providing context for acceptability.
What if the target mean equals the observed value?
The SDI equals zero, indicating perfect agreement.