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Precision is a term that describes the level of repeatability of measurements. When collecting a group of data, either by measurement or through an experiment of some kind, the precision describes how close together the results of each measurement or experiment are going to be.  Precision is not the same as accuracy. Accuracy measures how close experimental values come to the true or theoretical value, while precision measures how close the measured values are to each other. It is possible for data to be accurate but not precise or to be precise but not accurate. Accurate measurements are close to the target value but may not be close to each other. Precise measurements are close to each other, whether or not they are close to the target. The word “precision” does not have a single meaning. You can represent precision using several different measurements. You need to decide the best one.  Range. For small data sets with about ten or fewer measurements, the range of values is a good measure of precision. This is particularly true if the values appear reasonably closely grouped. If you see one or two values that appear far from the others, you may wish to use a different calculation. Average deviation. The average deviation is a more accurate measure of precision for a small set of data values.  Standard deviation. The standard deviation is perhaps the most recognized measure of precision. Standard deviation may be used to calculate the precision of measurements for an entire population or a sample of the population. Very often, investigators will report data by giving the mean of the measured value, followed by a statement of the precision. The precision is shown with a “±” symbol. This provides an indication of precision, but it does not clearly explain to the reader if the number following the “±” symbol is a range, standard deviation, or some other measurement. To be very clear, you should define what measure of precision you are using, either in a footnote or parenthetical note. For example, for one series of data, the result could be reported as 12.4±3. However, a more explanatory way to report the same data would be to say “Mean=12.4, Range=3.”
Use the word precision correctly. Choose the best measure of precision. Report your results clearly.