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Accuracy, Precision and Error

Author: Cecil McIntosh

Lesson 1 - Accuracy and Precision

Focus on Accuracy and Precision, and how to calculate the Accuracy and Precision of measurements.

Error - Types of Error and how they affect measurements

Terms and Equations

Terms And Equations


Accuracy  – How close a measurement is to the actual or expected value.

Precision – How close a set of measurements are to one another. 

Error –  The difference between the actual or expected value, and the measured value.

Absolute Error  =  Actual value – Measured value

Relative Error =  (Actual value – Measured value) / (Actual Value)

% Error –  Relative Error X 100%

Deviation – The difference between a measurement within a set and the mean of that set of measurements.

Deviation = Measurement  - Mean

Average Deviation -  The average of the Deviations of all measurements within a set.

Systematic Error – Error that impacts all or a large portion of measurements in a set uniformly, or by some type of trend.

Random Error – Error that occurs sporadically, without any sort of trend or predictability.

Offset error – is when a machine is not calibrated or set to measure zero, when the sample is at zero.

Multiplier error -  is an equipment malfunction where it amplifies changes in readings, or has a diminished sensitivity to change.

Source: Chemistry Quick Review of Experimental Error. Anne Marie Helmenstine, Ph.D.,,

Practice Problems