Chemical Education Today
Letters Including (0,0) as Experimental Data The Kim and Burkart response (1) to a letter (2) discussing the visualization of statistical concepts includes the short statement, “one often tends to use the origin point (0,0) in the data. However, whether that is best practice or not is entirely arguable”. Let us get this straight. It is never, ever defensible to invent data and add it to actual observations in fitting or data analysis. I am sure none of your correspondents would countenance it for a second in any other context; why should they do so here? The argument that a zeroed instrument is expected to provide a point at (0,0) is specious and misleading. What the experimenter expects the experiment to do is not—ever—an acceptable substitute for the observation itself. An actual observation at zero concentration on a zeroed instrument is practically never zero except by chance or censoring (e.g., by the instrument or analyst reporting zero if the response is below some arbitrary threshold—in which class I include the detection limit). If you want a data point at concentration zero, get a data point for concentration zero. Do not invent it, and do not pretend that inventing it is even remotely good practice, even—or especially—in front of your students. Literature Cited 1. Kim, Myung-Hoon; Burkart, Maureen. J. Chem. Educ. 2008, 85, 635. 2. de Levie, Robert. J. Chem. Educ. 2008, 85, 635.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2009/Jul/abs809_1.html Keywords Full text (HTML and PDF) with links to cited JCE articles Stephen L. R. Ellison Bioinformatics and Statistics Laboratory of the Government Chemist Queens Road Teddington, Middlesex TW11 0LY, UK
[email protected] Response to Including (0,0) as Experimental Data It was not our intention to imply that data may be invented or manipulated; such a practice is intolerable. If we had given more explanation with the statement in our previous letter “one often tends to use the origin point (0,0) in the data. However, whether that is the best practice or not is totally arguable depending on various conditions of the measurements”, it may not have incurred criticism from Ellison. Thus we believe that he provided us with an opportunity for clarification. When using an absorption spectrophotometer, it is necessary to use a blank to zero the instrument. The appropriate blank may be one of two choices, as appropriate: (i) water or other solvent or (ii) a background solution containing all solutes except the analyte. When using a single-beam instrument, a measurement must be taken at zero concentration so that there is a data point at zero concentration; this is necessary owing to possible fluctuation in the background signal. However, if a double-beam spectrophotometer is used, it is not necessary to take a measurement at zero concentration. For most of the double-beam spectrophotometers, background currents are instantly compensated. Measuring absorbance at zero concentration after zeroing with the blank is redundant. An exception to this is a case where water (or other solvent) is used as a blank instead of the background solution (described in ii) when zeroing an instrument: in this case, a measurement at zero analyte concentration must be obtained because of a possible absorption from other solutes present in the background solution. Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2009/Jul/abs809_2.html Keywords Full text (HTML and PDF) with links to cited JCE articles Maureen Burkart and Myung-Hoon Kim* Science Department Georgia Perimeter College–Dunwoody Campus Dunwoody, GA 30338 *
[email protected] © Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 7 July 2009 • Journal of Chemical Education
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