Standard Deviation Index (SDI) Calculator

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The mean of your laboratory's replicate measurements for the analyte being assessed. Also called the test mean or your result.
The mean calculated from all peer laboratories in the proficiency testing program for the same analyte and concentration level.
The standard deviation of the peer-group results. Must be greater than zero. A tighter group SD means a more demanding proficiency standard.
Standard Deviation Index (SDI)Acceptable
0.5

Signed bias in units of the group standard deviation

Absolute SDI0.5
Percentage bias0.13%
Absolute bias1
Performance bandAcceptable
0.5
Acceptable<1Investigate1-1.5Marginal1.5-2Unacceptable2+

SDI = 0.50 - Acceptable.

  • Your laboratory mean (9) is above the consensus group mean (8), with a raw bias of +1.0000 and a percentage bias of 12.50%.
  • An SDI of 0.50 falls within the acceptable range (|SDI| <= 1.0). No corrective action is needed, but continued monitoring is always good practice.
  • SDI is signed: a positive value means your lab reads high relative to peers; a negative value means your lab reads low.

Next stepTrack your SDI over time across multiple surveys. A single acceptable SDI is reassuring; a consistent trend away from zero can reveal a slow drift even within acceptable limits.

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