Two commonly discussed measures of observational error are accuracy and precision. Accuracy refers to how close measurements are to the true or accepted value whereas precision refers to how close measurements are to each other. The two terms tend to be used in conjunction with one another but are independent concepts. In the figure, accuracy is related to how close the dots are to the center of the target. Precision is depicted by how close the dots are to each other. Dots with a mean at the center of the target but spread about are accurate but not precise. Dots that are tightly grouped together but are away from the center of the target are precise but not accurate.
Errors in accuracy may be a characteristic of the test or devise being used. Precision is related to reproducibility and repeatability in which repetition of the measurements under the same conditions will result in the same or similar results. Increasing the sample size or the number of measurements performed will improve the precision but not the accuracy.
Understanding the accuracy and precision of a device has implication in research. The goal is to use a device that is both accurate and precise, but that is not always available. Knowing the accuracy and precision of a device can help with interpreting results and decision making, especially with imperfect tests.