The geology of the deposit was covered by him in an article entitled "An Unusual Fluorspar Deposit" published in the Engineering and Mining Journal, for September, 1928. LENHERSCHWERIN VIcronv FLUORSPAR MININGCOMPANY EL~AEETRTOWN, ILLINOIS
OBJECTIVE TESTS IN ORGANIC CHEMISTRY The 1940-1941 Series of the Cooperative Objective Tests in Organic Chemistry are now in the process of preparation. It is believed that the present series will meet the need of the average instructor of organic chemistry somewhat better than any of the tests of this series thus far prepared. It is hoped, furthermore, that those who elect to use these tests during the coming school year will cooperate with us to the extent that this series may be standardized and the validity of each item determined. One page is devoted t o each of twenty-six topic examinations in organic chemistry, and three pages each to the final examinations for the t i r t and second semesters, making a total of thirty-two mimeographed pages to a set. These sets are available to instkctors and graduate students or research workers in lots of five or more a t twenty cents each. Single sets are thirty-five cents. Those using these tests in their classes will be provided with a key for convenience in grading. Anyone who is interested in examining or using the 1940-1941 Series of Cooperative Objective Tests in Organic Chemistry may place his order with Ed. F. Degering, Chairman, Cooperative Objective Tests in Organic Chemistry, Purdue University, Lafayette, Indiana. Ep. F. DEGERING P ~ U UNIVERSITY E LAFAYETTE. INDIANA
"THE EFFECT OF THE NATURE OF THE COURSE ON ACHIEVEMENT IN FIRST-YEAR COLLEGE CHEMISTRY" To the Editor DEARSIR: Clark might be able to philosophize upon the socalled "conclusion-inevitable" that he makes in the article entitled, "The Effect of *Nature of the Course on Achievement in First-Year College Chemistry,"' but no statistician or scientist would allow him to base his conclusions upon the numerical gymnastics that he
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C-, "The effect of the nature of the course on achievement in first-year college chemistry," J. C m . EDUC.,16,510-11 (Nov.. 1939).
describes. Of course, it is possible that the experimenter may have made many other computations without bothering to record them in his paper. Certain additional data could certainly prove useful, but this critic will naturally have to assume that all of Clark's work is described in the report. Little does Clark apparently realize how correct he actually is when he says, "Perhaps the reader should be cautioned against indiscriminately applying the above conclusions to other groups of students." (1) Clark attempts to compare the gains made by students in the elementary group and the advanced group by determinimg their "average gain". He obtains this gain by "end-test percentile minus pre-test percentile." By chance, the particular groups with which he was dealing (those having had high-school chemistry credit) actually increased their percentile ranks, thereby allowing him to ohtain positive numbers rather than negative ones. Percentile ranks are similar to rank order in which i t is easy to understand that a class of seventy-five must have seventy-five rank orders represented hoth in the pretest and in the end-test. The various changes both plus and minus that occur must always become equal to zero algebraically. A person holding his own must actually be gaining in true score and achievement if the class as a whole is learning anything. If a group, as was the case here, actually gains in rank order or percentile, it merely means that they are somewhat more select in their rate of increase than the others in that same group. Clark compares these rates of gains with the rates of gains of another group that apparently has not been equated in any way. These figures are meaningless arid misleading. (2) Clark'finds a correlation of 0.88 between the pre-test and end-test percentiles for the control group (elementary chemistry). In the 6rst place, percentiles represent a rectilinear distribution rather than a frequency distribution which might,be more similar to the normal curve. Equations for computing correlation are not designed for rectilinear distributions and therefore the data are less meaningful than might be supposed. (3) However, after finding this correlation of 0.88, Clark sets up a regression (for this elementary group) and gives i t as
XI
=
0.98 Xs f 17.12
With this, he can predict the end-test ranks from those on the pre-test. Accordingly, "end-test percentiles were then predicted for all members of the control group and a correlation of 0.88 was actually found between the predicted and the earned values." All Clark seems to have proved by this useless manipulation is that his arithmetic has apparently been correct both times and that the old truth, things equal to the same thing are equal to each other, is still as true as ever. (4) "By means of this same regression equation," for the clark then predicted the end-test students in the advanced group. The real meaning of
