Posted by Sten Westgard, MS
Here's a question that came in about setting the control limits (or range) for a test:
"for some assays we're using this formula: actual SD * 3 and then divided by 2 plus or minus the mean is this acceptable or not because when we use that give us abit wider range than using the mean plus minus 2SD."
When we asked for an example, we got this data:
Manufacturer Data: SD = 22.5, Mean = 224Actual Data: SD = 8.79, Mean = 223
"We're multiplying ourSD (8.79) by 3 and then we divide it by 2 to give us the new SD which is 13 (8.79*3/2 = 13). Then we multiply this new SD 13 by 2 to give us the real 2 SD range which is 26. So our range is now 197 - 249. Are we following the right way or not?"
The answer, after the jump...
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(Or, if only some surveys are based on accuracy, then what are the other surveys based on?)
Posted by Sten Westgard, MS
There's an article that appeared in the October 2010 issue of CAP Today that probably didn't get enough attention. It covers a subject that's been gnawing at us for a while:
Accuracy-based Surveys carve higher QA Profile, by Anne Paxton
For those of you who thought all proficiency testing was "accuracy-based", this article may give you a bit of a shock. In fact, most PT surveys - indeed most EQA programs and even peer-group programs - are not based in accuracy. Instead, those surveys are only based on "consensus."
What's the difference, What does it mean - and how did it come to be this way?
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