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**Accuracy and Precision**

Measurements are qualitative or quantitative. Qualitative measurements give results that are descriptive and non-numerical. Example: Quantitative measurements give results that are definite, usually as numbers and units.

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**Accuracy and Precision**

Measurements work best when they are accurate and precise Accuracy is a measure of how close a measurement comes to the actual or true value of whatever is measured. To evaluate the accuracy of a measurement, it must be compared to the correct value. Precision is a measure of how close a series of measurements are to one another. The precision of a measurement depends on more than one measurement.

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**Accuracy and Precision**

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**Accuracy and Precision**

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**Accuracy and Precision**

In this picture, all of the darts land on the bulls-eye which illustrates good precision and accuracy. In this picture, all of the darts land near each other, but away from the bulls-eye which illustrates good precision, but poor accuracy.

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**Accuracy and Precision**

Here the darts are not close to each other, but are close to the bulls-eye indicating poor precision, but good accuracy. Finally, the darts are not close to each other or near the bulls-eye indicating both poor accuracy and poor precision.

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**Accuracy and Precision**

Sometimes there is a difference between the accepted value and the experimental value. This difference is known as error. Error = accepted value – experimental value Error can be positive or negative depending on whether the experimental value is greater than or less than the accepted value.

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**Accuracy and Precision**

Often it is useful to calculate relative error, or percent error. Percent error = error x 100% accepted value The percent error will always be a positive value.

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