What is the Standard Deviation Index?
The Standard Deviation Index (SDI) is a quality-control statistic used to compare a laboratory’s test result against the mean of a peer group of similar instruments or methods, expressed in units of standard deviation.
The formula
SDI = (Your Result − Peer Group Mean) / Peer Group Standard Deviation. An SDI near 0 indicates the result is close to the peer group average. Many quality-control programs flag results with an absolute SDI greater than 2, and treat values beyond 3 as a significant deviation worth investigating.
This calculator is for general quality-control education and planning purposes, and does not replace an accredited laboratory’s own QC procedures.
Last reviewed August 2026