A FIRST EXPERIMENT 
33 
est in the second test was eighth in production, and the 
girl who was second was fifth in production, etc. There 
may be a difference all along the line, showing that there 
was a lack of agreement or a low correlation between the 
performance of the girls in this test and their performance 
at their regular work. 
In every case, the degree of agreement or correlation 
between tests and production is determined, not by guess 
work, but with mathematical exactness, by means of the 
method already described and certain simple formulae dis 
cussed in the Appendix under the heading Correlations . 
It would be too cumbersome to go into further details on 
this subject here. Suffice it that the degree of correspond 
ence between two sets of rankings, the ranks of a group 
°f subjects in production and their ranks in a given test, 
can vary between plus i.oo and minus i.oo. Plus i.oo 
is a perfect correlation. Minus i.oo is an absolutely 
negative correlation. Plus .70 or .60 is considered good 
for shop and factory conditions. A test which, under 
factory conditions, shows a correlation with production 
°f .40 or more is considered a valuable or a significant 
test. 
The method of computing the value of tests just de 
scribed was applied carefully to each of the tests given in 
this experiment with the following results: 
Table of Correlations 
Card sorting 5® 
Tapping x 4 
Cancellation ^3 
Easy directions x 4 
Number group checking 7 2 
Accuracy 3 8 
Steadiness 2 4