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supl001 - 161 Supplement SUPL 001 001 supplement Analysis...

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161 Supplement SUPL- 001 Analysis of Experimental Reliability SUPL-001 c o n c e p t s SFMJBCJMJUZ!t!QSFDJTJPO!t!BDDVSBDZ!t!SFQSPEVDJCJMJUZ!t!JO. EFQFOEFOU!NFBTVSFNFOU!t!JOUSJOTJD!QSFDJTJPO!t!BWFS. BHF!)>!BSJUINFUJD!NFBO!>!NFBO*!t!BWFSBHF!EFWJBUJPO!t! BCTPMVUF!WBMVF!t!QFSDFOU!t!QFSDFOU!EFWJBUJPO!t!SBOEPN! FSSPST!t!TZTUFNBUJD!FSSPST!t!IPNPHFOFPVT!t!IFUFSPHF. OFPVT!t!TJHOJm !DBOU!m !HVSFT Prepared by R. F. Schneider (Revised 3/09) supplement 001 An essential part of any quantitative labora- tory measurement is an understanding of the reliability of the measurement. Powerful statistical methods exist for the analysis of reliability. The exercises conducted in the in- troductory chemistry laboratory will utilize procedures and apparatus with well-known limits of accuracy and precision. Normally, measurements will be replicated several times in order to establish the reliability of those quantities reported as conclusions of exercises (and on which a substantial part of the earned grade will depend). The number of repetitions of a measurement will typi- cally be relatively small (3–4 repetitions). Simple statistical concepts suffi ce to repre- sent the reliability of measurements that are replicated small numbers of times.
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162 Supplement SUPL- 001 Measures of Reliability We use two quantities to characterize the reliability of a series of experimental mea- surements— accuracy and precision . Accuracy measures the extent to which mea- surements agree with a known or “true” value. Precision measures the consistency of measurements with each other, independent of their accuracy. Figure 1-1 demonstrates accuracy and precision. It shows the result of a series of at- tempts to hit the bull’s-eye of a circular target with a bow and arrow. The objective of this exercise (i.e., its known value) is to have each arrow strike the center of the target. Concentric circles on the target mark areas which are successively further from the objective. The distance from the center of the bull’s-eye represents the error in an attempt. Archer 1 shows great consistency in the part of the target that the arrows strike, but they are all distant from the center of the target. The data are precise, but not accurate. Archer 2 shows inconsistency in the location of the arrows on the tar- get, but there are about as many of them on the right as on the left and on the top of the target as on its bottom. This archer demonstrates some measure of accuracy, but with low precision. Archer 3 demonstrates both accuracy and precision. How does accuracy play a role in experiments whose results are qualitative ? If on the basis of a series of experimental results, a sample is asserted to be potassium iodide, the assertion is accurate (i.e., true ) if the material was, in fact, potassium iodide. If the material was another substance, the assertion is inaccurate, which in this case means false . If the data from which the identity was concluded does not permit distinguish- ing potassium iodide from sodium iodide, asserting that the substance may be one OR the other would be an accurate (i.e.,
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