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.