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RPD Calculations for non detects
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 RPD_esdat.JPG (169kb) downloaded 9 time(s).
I have noticed what may be an error when producing a Relative Percentage Difference table in ESdat. As shown in the attached image, the RPD calculated for Total PAHs is 67% between values of 0.1 and <0.05. The result of 67% difference assumes that the ‘less than’ symbol has been ignored and 0.05 is used in the calculation. However, as can be seen in the RPD for BaP TEQ (half), the RPD is 0% between values of 0.1 and <0.5. Given the same logic, an RPD of 133% would be correct if the ‘less than’ sign was ignored here. It seems that generally, any ‘less than’ values are considered to be 0, generating an RPD of 0%, so I’m just wondering why we now have an RPD of 67%? Edited by moderator Thursday, November 5, 2020 3:26:28 PM(UTC)
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Groups: Registered, Administrators Posts: 327  Was thanked: 29 time(s) in 29 post(s)
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Hi Emily,
Thanks for your post.
ESdat is calculating the RPDs correctly. Please have a look at this forum post for some worked examples: https://www.ESdat.net/forum/posts/t33-RPDs-from-a-non-detectable-analyte
What you are observing is actually two different scenarios and not comparable (this is explained in the linked post).
The first, where an RPD is calculated, is because the ND is less than the detect (<0.05 and 0.1), uses the EQL to calculate the RPD.
The second, where the detect is less than the ND (0.1 and <0.5), does not calculate an RPD.
Thanks,
Nick
Edited by user Thursday, October 22, 2020 8:09:35 AM(UTC)
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