How this calculator works
SDI normalises a result or laboratory mean against a specified peer-group mean and peer standard deviation. A positive index is above the peer mean and a negative index below it. Rearranged modes use the same equation and reject an ambiguous or impossible missing SD. The peer SD is a measure of peer spread, not the standard error of the peer mean. Interpretation depends on the peer group and quality programme.
Formula
SDI = (result - peerMean)/peerSD; result = peerMean + SDI × peerSD; peerMean = result - SDI × peerSD; peerSD = (result - peerMean)/SDI.
Worked example
A result of 110 against peer mean 100 and peer SD 5 gives SDI = (110 - 100)/5 = 2. An index of -1.5 against mean 20 and SD 2 corresponds to result 17.
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Frequently asked questions
Is an index an automatic pass or fail?
No. Evaluation limits come from the applicable proficiency or quality-control programme. This tool reports only the standardised difference.
Is this a patient diagnostic score?
No. It is a statistical comparison with a chosen reference distribution and does not establish a clinical diagnosis.
Can I solve SD when the index is zero?
No unique SD follows from a zero index. Equal means allow many positive SDs, while unequal means are inconsistent with zero.