What is the SDI and how do I use it?

The SDI, Standard Deviation Interval (or Index) is defined as:

Your Result - Comparison Group Mean 

Comparison Group Standard Deviation 

The SDI describes how far a proficiency result is from the comparison group mean. The closer the SDI is to “0”, the closer the result is to the group mean. An SDI of -1.0 means that your laboratory's reported result is one standard deviation below the mean of the group; an SDI of +2.0 means that your reported result is two standard deviations above the mean. 

A review of the SDIs can assist with detection of random or systematic errors before they are large enough to be detected by quality control results. Random errors are caused by unknown and unpredictable events, and occur without any real pattern. Systematic errors are continuous and affect all results equally. Systematic errors are indicated by a trend or shift, and are commonly due to mis-calibration. 

In a broad sense, when looking at the SDIs for a group of five proficiency samples the numbers should be both positive and negative. Systematic errors are indicated by trends in the data; if the SDIs are all positive numbers (i.e. your proficiency results are all above the group mean) your instrument's calibration could be drifting to the high side. Conversely, if your SDIs are all negative numbers your calibration could be drifting to the low side. Significant systematic error must also be considered if two or more SDIs exceed the same limit and the average of the five SDIs exceeds +1.5 or -1.5.

 

If you have questions about SDIs or would like further assistance, please contact Technical Support at [e-mail] or 800-333-0958.