An Experiment for htroducing Statistics to Students of-~nalyticaland Clinical Chemistry Richard A. Paselk Humboldt State University, Arcata, CA 95521 Statistical tools are extremely important in modern clinical and analytical chemistry laboratories. The following experiment was designed to introduce a variety of statistical tools and concepts to students in a short course on instrumental analysis. This course is primarily for majors in medical technology. Since these students will work in laboratories where both the accuracv and the vrecision of ouantitative laboratorv data can be a matter of liie and death;it is critical that the; understand the nature of these concevts and their anvlica.
.I.he experiment imply involves making a series
of repetitive dilutions with a commercial diluter and subsequ~!ntly determining their concentrations using a spectrophotometer such as a Spectronic 20. Each student prepares a dilute standard solution of bromcresol green by volumetrically transferring 5.00 ml of stock dye solution (1.000 gll; the exact concentration is not important, but it should be measured to four significant figures) into a 100-mlvolumetric flask and diluting to volume with water. The absorbance of this solution, which will be used to calculate the concentrations of all subsequent solutions, is then measured in triplicate a t 615 nm, and the results averaged. (I generally suggest that the students make all measurements using a single cuvette, rinsing with sample before each measurement. A second. matched cuvette mav then he filled with water and used as; blank for periodic iistrument checks.) The students then prepare five sets of ten replicate dilutions each of the stock dye solution using commercially available diluters (e.g., Labindustries "Repipet"). A total of five sets are prepared by each student, with the diluters being reset between sets. Usingone diluter, two sets are prepared a t a dilution of 250 ~1 to 10.0 ml, audtwo sets are prepared a t a dilution of 503 pl to 10.0 ml. The fifth set is made with a second diluter using one of these dilutions. Absorbances are then measured and recorded to three significant figures. The students are now asked to make a variety of calculations beginning with the means and standard deviations (expressed as concentrations) a t each dilution. Sample calculations are required so that students learn more than simply
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Journal of Chemical Education
which button to push on their calculators. They are then asked t o assess the accuracv to the standard solution. and . (relative . making the assumption that the sportrophotorneter is error free) and vre~:isionof their diluter nt each volume. The students then note how precisely the diluters were reset by comparing means at a single setting. Confidence intervals are next calculated for one data set a t each dilution. Data sets are also compared for accuracy using the t-test and for precision using the F-test. Two different diluters are compared with the t-test a t one dilution, while the F-test is used to compare the precision obtained by two different students using the same diluter a t the same dilution. Finally, the students are asked to set up a quality control chart, the Levey-Jennings chart', using one of their data sets. In completing the handout for this experiment, the students are also&kedto comment on their calculated results and to discuss the statistical tools utilized. The students are given a week following the completion of the exveriment in which to finish the calculations and to answer the handout questions, during which time we also complete our brief coverage (23h) of these topics in lecture. Even though most of our students have no background in statistics, they are largely able to accomplish this exercise successfully, using the statistical materials in their textsI.2 and a recommended refe~ence.~ The students show a .. eood .. erasv. of this material on subsequent exams, and, more telling, graduates report how helpiul this introduction is later durine internshivs and when taking national exams. I would be happy to send copies of our instruction sheets and write-up forms along with sample calculations, upon request.
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Henry, John B.. 'Clinical Diagnosis and Mmgement by Laboratcay Methods," 17th ed., W.B. Saunders Company, 1984, Teitz, Norbert W., "Fundamentals of Clinical Chemistry," 2nd ed., W. B. Saunders Company, 1976. Henry, R. J.. and Dryer, R. L., in "Standard Methods of Clinical Chemistw," Vol. 4. (Editor: Seliason. D.). Academic Wess. New York.
