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Unformatted text preview: CHAPTER 11 MEASUREMENT AND DATA PROCESSING (IB TOPIC 11) SUMMARY Uncertainties and errors Random uncertainties (or errors) arise mostly from inadequacy or limitation in the instrument or the way a measurement is made. Random errors make a measurement less precise, but not in any particular direction. These are written as an uncertainty range, such as 44.20 ± 0.05 cm3. Systematic errors are due to identifiable causes, and arise from flow in the instrument or errors made in taking a measurement such as an incorrect calibration of a pH meter or reading the top rather than the bottom of the meniscus. Systematic errors always affect a result in a particular direction (always smaller or larger) unlike random errors. Random uncertainties can be reduced by repeating readings; systematic errors can not reduced by repeating readings. Precision If an experiment is repeated many times, the precision is a measure of how close the repetitions will be to each other. The precision or reliability of an experiment is a measure of the random error. If the precision is high then the random error is small. Accuracy The accuracy of a result is a measure of how close the result is to some accepted or literature value Accuracy is a measure of the systematic error. If an experiment is accurate then the systematic error is very small. A measurement can have a great degree of precision, yet be inaccurate such as if the top of a meniscus is read in volume readings using a pipette or a measuring cylinder instead of he bottom of the meniscus. Significant figures The number of significant figures in any calculation should be based on the number of decimal places/significant figures in the data based on the following simple treatment: • In addition and subtraction: Add absolute uncertainties • In multiplication, division and powers: Add percentage uncertainties • If one uncertainty is much larger than the others, ignore the other uncertainties and estimate the uncertainty based on the larger one using the rules above. © IBID Press 2007 1 ...
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This note was uploaded on 09/23/2010 for the course CS 001 taught by Professor Jix during the Spring '10 term at Riverside Community College.

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