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In science, we make lots of measurements, and every measurement has some error. We can express the amount of error in a given set of measurements by discussing the accuracy and precision of the set of measurements.
Accuracy is defined as how close the average of a set of measurements is to the accepted value of the measurement. Precision is defined as how closely grouped a set of measurements is.
See the image below for a visual demonstration of accurate and precise measurements. The target in the upper left-hand corner of the image represents an accurate set of measurements because the AVERAGE of the locations of the x's is located at or near the bull's eye. However, the same set of measurements is NOT precise because the x's are NOT closely grouped. The target in the upper right-hand corner of the image represents a precise set of measurements because the x's are really close together. However, they are NOT accurate because the average of their locations is not at or near the bull's eye. The lower left-hand target represents both an accurate and precise set of measurements, and the lower right-hand target represents a set of measurements which is neither accurate nor precise.
Accuracy is defined as how close the average of a set of measurements is to the accepted value of the measurement. Precision is defined as how closely grouped a set of measurements is.
See the image below for a visual demonstration of accurate and precise measurements. The target in the upper left-hand corner of the image represents an accurate set of measurements because the AVERAGE of the locations of the x's is located at or near the bull's eye. However, the same set of measurements is NOT precise because the x's are NOT closely grouped. The target in the upper right-hand corner of the image represents a precise set of measurements because the x's are really close together. However, they are NOT accurate because the average of their locations is not at or near the bull's eye. The lower left-hand target represents both an accurate and precise set of measurements, and the lower right-hand target represents a set of measurements which is neither accurate nor precise.