FEBRUARY, 1955
AN ACCURACY-DETERMINATION CHART PETER LOTT University of Connecticut, Storrs, Connecticut
IN COURSES in quantitative analysis, students report their results in the percentage present of a certain constituent. The students are usually graded on the basis of the parts per thousand by which they differ in their report from the known value of the sample. Thus, if a student should report 44.22 per cent and the standard unknown analysis of the sample were 44.00 per cent the deviation would he .22 parts in 44.00. To determine the error in parts per thousand, the following calculation is generally performed: ==L 44% 1000
where x is the deviation in parts per thousand. Grading the reports for a whole class involves a considerable amount of slide-rule calculation and a large chance for error is present, especially in misplacing a decimal point. As the student is usually not told what the true value is for the sample, a slide-rule mistake on the part of the instructor would seriously handicap the student. This danger is eliminated by using the accuracy-determination chart described below, and considerable time is saved. The chart shows immediately the parts per thousand by which the student varies in his analysis. All one must do to read the chart is to connect the proper marks on the "deviation from true value in yo" scale t o the "percentage of constituent in sample" scale with a ruler. The intersection of the ruler with the diagonal line in the center gives the parts per thousand directly. Thus, in the example above in which the student reported 44.22 per cent and the true analysis was 44.00 per cent, the difference of 0.22 per cent (on the left hand scale) is connected with 44 per cent (on the right hand scale). The intersection of the ruler with the diagonal l i e gives the required answer of five parts per thonsand. The chart is best read with a transparent plastic ruler. It might also be conven-
ient for the grader to pencil in the grade intervals parallel to the parts per thonsand scale: Thus 4-6 parts per thousand would he equivalent t o a grade interval of 85, 6-8 parts per thousand to a grade interval of 75, etc. ACKNOWLEDGMENT
The author wishes t o thank Dr. Kuang Lu Cheng for his interest.
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A C C U R A C Y DETERMINATION C H A R T
