Precision and accuracy in liquid handling

Part 1: How to evaluate and optimise the performance of liquid handling systems

A key process in laboratory automation is dispensing liquid aliquots, for example using automated liquid handlers. Evaluating the performance of liquid handling systems is important for laboratory technicians as well as for manufacturers of liquid handling technology. The benchmarks for evaluating dispensing performance are precision and accuracy.

What is the difference between precision and accuracy?

When a liquid handler is dispensing a run of liquid aliquots, the respective liquid volumes will always deviate slightly from the intended target volume. Deviations can occur in terms of precision and in terms of accuracy.

Precision is a measure of the degree of variation of the individually measured volumes relative to each other within one dispensing run. It is a measure of volumetric reproducibility when dispensing liquid aliquots. It is often expressed in terms of the coefficient of variation (CV), which is defined as the ratio of the standard deviation to the mean value of a respective run.

Accuracy describes the deviation of the actual from the target dispensing volume. For example, if the user wants to dispense a reagent with a target volume of 100 nl and the dispenser delivers an actual volume of 103 nl, the accuracy is +3 nl or +3%.

Put simply, precision describes the random error or closeness of the respective aliquot volumes. Accuracy describes the systematic error or how close the mean of the aliquots is to the target volume.

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