A Poor SDI (Sigma Deviation Index) is indicated when SDI value exceeds what threshold?

• A. >1
• B. >2
• C. >3
• D. >4

Answer: (B)

SDI shows how far a lab’s result is from the peer group mean in units of the peer SD. You compute it as (observed value − peer mean) ÷ peer SD. When this value is greater than 2, the discrepancy is more than two standard deviations away from the group mean, which is unlikely to occur by chance if the lab is in agreement. In QA terms, that level of difference flags poor agreement and calls for investigation.

For example, if the peer mean is 100 with a standard deviation of 5, an observed value of 112 gives an SDI of (12/5) = 2.4, which is above the threshold and considered poor.

Choosing a threshold above 2 rather than 1, 3, or 4 balances detecting real disagreements without over-flagging typical variation; >1 would flag too many results, >3 is too strict for routine QA, and >4 would rarely occur.