Volltext: Elementary course in psychological measurements (4)

Elementary course in psychological measurements. 
pass smoothly among the dots. Owing to the fact that the number of 
experiments was only io instead of an infinite number, the actual curve 
will differ from the theoretical one ; with ioo or 1000 experiments it 
would approach the theoretical one more closely. 
To compare the degrees of sensitiveness of different persons, two 
quantities can be used : either the disc that corresponds to acertainper¬ 
centage, say 75%, or the percentage for a certain disc. To use either 
effectively a large number of experiments would be required ; the calcu¬ 
lation of the average of the two thresholds as found above gives a suffi¬ 
ciently accurate figure. It is evident that the higher the threshold the lower 
the sensitiveness ; in fact, probably the only proper definition of “sensi¬ 
tiveness” is “the reciprocal of the threshold.” “Reciprocal” of a 
quantity means i divided by that quantity. Thus, if two persons have 
thresholds of a' and a" respectively, their degrees of sensitiveness will be 
i/a' and i/a". 
Points to be noted. 
i. Note that the uncertainty of the threshold is indicated by the size 
of the probable error. 2. In the curves given in the figure, y is said to 
be a function of x. This is expressed by y =/(x). The particular 
curves assumed in the figure are taken from the science of probabilities. 
i. How would you define “ threshold ” so as to fit all kinds of sensa¬ 
tions? 2. What are presumably some of the mental conditions of the 
subject that contribute to his probable error? 
Exercise II.—Skin space. 
(Needed: two æsthesiometers, millimeter scale.) 
In its simplest form the æsthesiometer is a pair of dividers with blunt 
points. The points arc made of hard rubber, in order to eliminate sen¬ 
sations of temperature. 
A. Open the æsthesiometer several centimeters. Touch the two points 
simultaneously to the cheek in a vertical direction ; they will be felt as 
two. Repeat the experiment, reducing the distance between the points 
each time. To maintain the unprejudiced condition of the subject, in¬ 
sert occasional experiments with only one point touched to the skin. The 
subject is to state each time whether he feels one point or two. Con-


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