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Lesson 2 - Precision and Accuracy
The precision of a measurement is determined by the reproducibility of repeated measurements done under identical conditions. For example, if every member of a laboratory section determined the molarity of the same unknown acid, you might wish to know how precise the measured molarities were, i.e., in each case, how much did the measurement deviate from the mean (the class average) of the measurements (we will explore the concept of the mean in more detail in the next lesson)? If the precision is good you can usually state with some confidence that there are not large random or indeterminate errors influencing the results. Indeterminate errors are ones which are a natural consequence of the way in which the measurements were made.
Accuracy is a measure of the closeness of an experimental value to the true or accepted value of the property being measured. If the true value is unknown, then a highly precise value is usually considered to be accurate, but beware of systematic or determinate errors. These are errors which can be accounted for, and hopefully, avoided. For example, if all of your titrations were within 0.01 mL of one another, but your buret actually delivered volumes that were off by 10% over the range that you used, you would have a very precise measurement (few random errors) but not a very accurate one (significant systematic error).
Pressing the button above will take you to a figure showing four hypothetical distributions of data (idealized as "bell shaped curves"). The distributions are labeled a-d, and the vertical line in each figure represents the true value of the property being measured. Characterize each distribution in terms of precision and accuracy (using high and low as the characterizing terms). Use the response form to send your results to your Laboratory Instructor. |
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