What’s the difference between accuracy and precision?

by | Sep 1, 2021 | Reports

Workers at SVL prepare samples in lab

Have you ever wondered what the difference between accuracy and precision is? The classic example is this: I shoot five arrows from a hundred yards and they land either in or just outside of the bullseye.  The consensus from the gallery will be that I am a very accurate shooter.  The next shooter comes up and shoots five arrows and they land low on the target about two feet from the bulls eye but within inches of each other.  These arrows were not very accurate but the shooter showed amazing precision because all the arrows were grouped together.  

What can you take away from this classic explanation?  You may receive an analytical report where all the reproducible QC is spot on but the results don’t reflect the concentration of the analyte in your sample.

What’s the definition of accuracy?

The degree of agreement between an observed value and an accepted reference value.  Accuracy includes a combination of random error (precision) and systemic error (bias) components that are due to sampling and analytical operations; a data quality indicator.

What’s the definition of precision?

The degree to which a set of observations or measurements of the same property, obtained under similar conditions, conform to themselves; a data quality indicator.  Precision is usually expressed as standard deviation, variance or range, in either absolute or relative terms.

What’s an example of where the difference is important?

Nitrate has been deemed by the EPA to be immediately dangerous to life and health at concentration levels above 10 ppm.  You receive an analytical report that indicates that your sample contained nitrate at 9 ppm and also contained 9 ppm in the duplicate of your sample the lab analyzed.  How do you know this is accurate based solely on the analytical report – you don’t.  All the report proved was the analytical precision the lab reported was well done.  The report may not have been accurate, the actual concentration was 11 ppm when one looked at the error associated with the analytical run (which the lab did not provide you with).  So you really had a sample that contained nitrate at a level that the EPA states is dangerous.

How does that difference come up in SVL Analytical’s work?

When you look at your analytical report you should look at the QC section of the report and see how well the Laboratory Control Sample (LCS) was recovered.  For example you see that the LCS was recovered at 95%, you could then apply reduction to your sample.  You should remember though that the LCS is a known concentration put into pristine water, where there are no interferences with analytical recovery.  Your sample may contain matrix interferences that further adjust what is really in your sample.  So to be safe any analytical result close to levels the EPA sets should take into account accuracy over precision.

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