VALIDATION OF MEASURING INSTRUMENTS 175
the most popular forms. The Thurstone form is obtainable
from C. H. Stoelting Company, Chicago, Illinois; the Otis,
from the World Book Company, Yonkers, New York; and
the Toops, from H. A. Toops, Ohio State University, Colum-
bus, Ohio. These authors use modified formulas for ease of
calculation. The Thurstone Correlation Data Sheet is re-
produced in Figures 19 and 20.
Explanation of the Thurstone Correlation Data Sheet
(Figures 19 and 20).
1. The scatter diagram is plotted, always placing low scores in
variable x at the left of the diagram and high scores at the right,
and low scores in variable y at the bottom and high scores at the
top.
2. Estimate the class interval in which the mean of the x's oc-
curs and shade in the appropriate column. Do the same for the
¥’s, shading in the appropriate row. Correction for these esti-
mates will be made later.
3. Record in row L the frequencies of the measures in the col
umns.
4. Record in row M the deviations from the assumed mean in
units of class intervals.
5. Perform the algebraic processes called for in rows N and P.
6. Complete columns 8, 9, 10, and r1. These processes are
analogous to those in rows L, M, N ,and P.
7. The figures in columns 12 and 13 are obtained by summing
the values of each measure in the row multiplied by its deviation
from the assumed mean (as shown in row C ). For example, in
row D, oX1=0, 1 X3=3, 2 X2=4, 3 X2=6, and
4 X I == 4, the total being 17, as shown in column 1 3. The fig-
ures in row C are minus if they are to the left of the assumed
mean. Consequently, two operations are necessary for each row,
one for the measures to the right of the mean and one for the
measures to the left of the mean. In row E we have —1 Xr
==—TI, recorded in column 12, and § X 1 = 5, recorded in col-
umn 13.
8. Add algebraically the values in columns 12 and 1 3 for each
row and record in columns 14 and 1 5. For example, —1 and
-+ 5 =} 4, which is recorded in column 14.
9. Multiply the values in columns 14 and 15 by the correspond-
Ing values in column g. Remember that the values in column 9
