What does the standard deviation index (SDI) indicate?

The standard deviation index (SDI) is a statistical measure used to determine how far away a specific observation is from the mean in terms of standard deviations. It provides insight into whether a data point is typical or an outlier relative to the overall dataset. By calculating the SDI, one can assess whether a measurement falls within a normal range or if it significantly deviates from the mean, which is crucial in fields like laboratory medicine for interpreting results and making clinical decisions.

For example, an SDI of 2 indicates that the observation is two standard deviations away from the mean, suggesting it is relatively unusual. This type of analysis helps in quality control and evaluating the reliability of test results in a clinical laboratory setting, ensuring that deviations from the expected results are appropriately addressed.