ChE253K_Precision&Accuracy-Willson

ChE253K_Precision&Accuracy-Willson - Understanding...

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Understanding Precision and Accuracy CHE 353M What’s the difference between precision and accuracy? One of the worst things you can do in your lab report is to use the terms precision and accuracy interchangeably! Precision and accuracy are two very different issues as illustrated below: A measuring device may be remarkably precise, and at the same time, inaccurate. Precision describes the ability of a measuring device to provide the same readings for the same input. However, precision makes no implications about the accuracy of the measuring device. Accuracy describes how close the measured value is to the “true” value as established by some reference standard. You may take a series of readings that are scattered over a range, yet averaged together, they provide an accurate value.
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The following types of errors affect accuracy, but not precision: Using the ideal gas law in non-ideal conditions Incorrect measurement of an orifice diameter that is used in your calculations A dead-weight tester with weights not corrected for local gravity A scale that is not zeroed before taking measurements The following types of errors affect precision: The random nature of humans reading a gauge differently (parallax effect) Random fluctuations in a system Often, it is stated that an instrument must be precise to be accurate. Suppose that an instrument has scattered readings that average out to be the “true” value. This is accurate if you take many readings, but if a user was to read the device once, it is rather improbable that they will happen to read the “true” value. Thus if an instrument has a precision error, it is often deemed inaccurate. This is acceptable as long as one understands that
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