REPORT FOR ANALYTICAL CHEMISTS of checking analytical calculations. These two advantages of uniform spécifie weights are overshadowed by two disadvantages, namely, the introduction of bias (either conscious or unconscious) and the loss of a device for detecting systematic error in the procedure under test (22, p. 40). In the authors' opinion, uniform weights should never be used in cooperative work. It is permissible in most' cases to suggest a small range or two small ranges, one being about double the other. There are cases in which the range of weight must be prescribed by the test method itself. For example, in the determination of heat of explosion, density of loading— i.e., relationship of size of sample to size of the bomb—influences the \7alues obtained and must be held within narrow limits. Data Forms. Forms for the collection of data should be designed, tested by use in the source laboratory, and distributed in triplicate to the participants by the chairman. One form can then be returned to him for his report, and the other two retained for use by the participating laboratory. In addition to spaces for the recording of test data, these forms might also include provision for the noting of the following information: Assignment of operators Operator's time Total elapsed time Deadline date for return of data to chairman Remarks or suggestions on testing procedure Departures from specified procedures If the identities of participants are not to be coupled with their data, the chairman will assign code numbers to be used on data sheets and in his report. Committee D-13 of the American Society for Testing Materials has recommended a standard (although somewhat flexible) form for data collection (2). In the analytical panel work, the forms have varied widely depending upon the type of problem and a standard form has not been feasible. Examples of forms used in the first twelve round robins of the Analytical Panel may be seen in a collective report by Pierson (17).
Rounding Off the Data. Good rules for rounding off data for reports are conveniently available (1, 7, 19, 22) and may be applied by the coordinator of a cooperative test program in the final stages of report preparation. Rules for data collection in cooperative testing are different! The chairman should specify in his instructions exactly how many digits to the right of the decimal he wishes the analysts to report for each test value obtained. He should set this at the level of the digit that represents the last place the analyst can estimate. If the rounding off is done prematurely —at the operator level—there is a high probability that the greater part of the measures of precision will be thrown away, and that efforts put into the work will be completely wasted. Early rounding off is then to be rigorously avoided in interlaboratory work until the statistical evaluations have been made ; it is much better to have too many decimal places than too few (22, p. 7) ! Table I is a simple illustration of the complete loss of statistical measures of precision when one decimal place is dropped prematurely. The values of 18 replicate moisture determinations, 6 for each of three laboratories, were estimated and reported to the third decimal place in the table as shown. Observe that if the data were rounded off to two decimals—the place to which the results are customarily reported when they are to be used only for calculating percentage values of other ingredients to a "dry basis"—every replicate would have exactly the same value, 0.02%, all the means would be 0.02%, standard deviations for all three laboratories would be zero, and no statistical evaluations would be possible. With the values shown to three decimals, precisions obtained by laboratories A and Β are equal and are better than that of laboratory C, and the three means are different. The statistical sig nificance of these differences can be examined by appropriate tests.
1111111111111111111111111 Range 100 cm — Reads to 0.01 mm. Scale engraved in 0.5 mm direct on solid Ws inch cross section bar.
CATHETOMETER
Reference
Page 11 of the Ealing Catalog for complete description of this outstanding instrument. This is just one of the more than 300 instruments for the physical sciences offered exclusively by Ealing.
THE
EALING CORPORATION 35 University Road Cambridge 38 Massachusetts
Statistical Design a Baird Atomic Affiliate
Simple vs. Complex Designs. In Circle No. 33 on Readers' Service Card VOL. 3 1 , NO. 12, DECEMBER 1959
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