Accuracy Versus Precision Practice

Science involves measurement, and measurement is inherently full of error. The reliability of any measurement depends on lots of factors. The limited precision of the equipment being used and errors made by people using the equipment are only two possible sources of experimental error. The accuracy and the precision of a measurement or of a set of measurements can tell us how reliable the set of measurements is.

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A set of measurements can be accurate without being precise (see the upper left-hand corner of the figure below). A set of measurements can be precise without being accurate (see the upper right-hand corner of the figure below). A set of measurements can be neither accurate nor precise (see the lower right-hand corner of the figure below). Ideally, a set of measurements should be accurate and precise (see the lower left-hand corner of the figure below).

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__accuracy__of a measurement or of a set of measurements tells how closely the average of those measurements agrees with the theoretical (known or accepted) value of the quantity being measured. For instance, the accepted value for the boiling point of pure water is 100 degrees Celsius. If you use a thermometer to take several measurements of the temperature of boiling water, the accuracy of your set of measurements is related to how close the average of your measurements is to 100 degrees Celsius: the closer the average of your measurements is to 100 degrees Celsius, the more accurate your set of measurements.The

__precision__of a set of measurements tells how close the values of the measurements are to each other. For instance, if you take several measurements of the temperature of boiling water, the precision of your set of measurements is related to how close together all of your measurements are: the closer together your measurements are to one another, the more precise your set of measurements.A set of measurements can be accurate without being precise (see the upper left-hand corner of the figure below). A set of measurements can be precise without being accurate (see the upper right-hand corner of the figure below). A set of measurements can be neither accurate nor precise (see the lower right-hand corner of the figure below). Ideally, a set of measurements should be accurate and precise (see the lower left-hand corner of the figure below).

Question: Why do the x's on the target in the upper left-hand corner of the image represent an accurate set of measurements?
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