I. EMPLOYMENT PSYCHOLOGY
which are used when the number of cases is small, when the
divisions are alternative rather than continuous, or when the
measures are in terms of ranks.
The method of rank difference takes into account only the
positions of the measures in the series, and should be used
when the measures are expressed as ranks. There are two
formulas: Spearman’s method of rank differences:
(17) ptm
N(N2—1)
where D represents any difference in the rank of a person in
the two series of measurements, and IV represents the number
 of paired measurements; and his “footrule for correlation”:

6Zg
(18) R=1 TR
where g represents only differences in ranks where there is a
gain in the second series over the first. Both p and R may
be converted into 7 by use of tables provided in most textbooks
 on statistics, based on formulas derived by Pearson
(see Tables 5 and 6). The conversion of p into r has doubtful
 value since the difference will usually be much less than
the probable error of the correlation coefficient. For example,
 a p value of .48 has an r value of .50, but the probable
 error of a correlation coefficient of .50 with 30 cases is
.09. Since the error of the coefficient is so much greater than
the correction to obtain 7, it would have been just as well
to leave it as p. The Scott Company (164) published convenient
 tables to shorten the labor of calculating p. These
tables are reproduced in the Appendix as Tables 3 and 4.
Instructions for their use are given below.
INSTRUCTIONS FOR CALCULATING RANK DIFFERENCE COEFFICI-ENTS
 (ForMuULA 17) WITH THE USE OF TABLES 3, 4,
AND 5. (SEE REFERENCE 164).
1. Arrange each of the two series of measurements to be correlated
 in rank order. Every pair of measurements is thus repre-78