What Is Standard Deviation Index?
The standard deviation index, or SDI, says how many group standard deviations a result lies from the group mean. It is a z-score where the reference is a peer group, and it is widely used for laboratory comparisons.
Enter one result or several, the peer group mean and the group SD. You get the SDI, the difference, the percent bias, a reading of how far out the result is and, for several results, the mean SDI and how many fall beyond two SDI.
Below are the formulas, a worked example and answers on acceptable limits.
How to Use the Standard Deviation Index Calculator
- Enter your result.
- Enter the peer group mean and SD.
- Read the SDI and the reading.
- Enter several results to review bias.
Standard Deviation Index Formula
Standard deviation index
Percent bias
Standard Deviation Index Variables Explained
| Symbol | Meaning | Notes |
|---|---|---|
| SDI | Standard deviation index | |
| x | Your result | |
| x̄ group | Peer group mean | |
| s group | Peer group SD |
Standard Deviation Index Worked Example
A laboratory reports 104.2 and the peer group mean is 100 with SD 2.5. Then SDI = (104.2 − 100) ÷ 2.5 = 1.68, and the bias is 4.2 %. The result is between 1 and 2 SDI above the group, which many programs flag for attention.
With the example values loaded in the calculator above, the steps are:
- SDI = (result − group mean) ÷ group SD.
- For the first result: (104.2 − 100) ÷ 2.5 = 1.68.
- |SDI| = 1.68, which is between 1 and 2 sdi.
Result: Standard deviation index (SDI) = 1.68; Reading = Between 1 and 2 SDI.
Where Standard Deviation Index Is Used
Proficiency testing
Compare a lab with its peers.
Quality control
Track bias over time.
Method comparison
Judge a new instrument.
Teaching
Show a z-score in context.
Common Standard Deviation Index Mistakes to Avoid
- Use the group SD, not your own.
- Look at the sign over time.
- Check your program’s limits.
Standard Deviation Index Terms and Definitions
- Peer group
- Laboratories or instruments measuring the same sample.
Standard Deviation Index FAQs
What is the standard deviation index?
The number of group standard deviations a result lies above or below the group mean.
How do I calculate SDI?
Subtract the group mean from the result and divide by the group standard deviation.
What is an acceptable SDI?
It depends on the program; values within ±1 are often considered good and beyond ±2 are usually reviewed.
How is SDI different from a z-score?
The calculation is the same; SDI is the name used when the reference is a peer group.