94 ROBERT M. YERKES. 



centage value of the mean variation in terms of the mean, I see 

 no reason why we should not call it the relative variability, in 

 contrast with the absolute variability. Thus the confusion with 

 Pearson's quantity which will inevitably result from the use of 

 variation-coefficient can be avoided. 



Myers' formula gives us precisely what we need for the direct 

 comparison of reaction-times, with respect to their variableness, 

 either to different stimuli or of different organisms. Strange to 

 say most investigators of the time relajtions of neural processes 

 have paid little or no attention to the variability of their results ; 

 none, so far as I know, have ever determined the relative varia- 

 bility throughout their work. It may be objected that those who 

 have use for the relative variability can find it for themselves since 

 the reaction-time and its mean variation are usually given. But 

 the value is far too important to be left half-way determined ; in 

 fact it is even more useful in most cases than the mean variation. 

 Every one who has had experience in dealing with reaction-time 

 results will admit that the reaction-time to a particular stimulus 

 has different meanings according to its variability, and that it is 

 never possible to compare reaction-times without considering this 

 value. It is clear then that no reaction-time statistics sJiould be 

 published without determinations of the relative variabilities. 



Conventionally we compare human reaction-times to visual, 

 tactual and auditory stimuli without noticing their variabilities 

 or the strength of the stimulus employed. Jastrow : in a table of 

 results, collected from the papers of many investigators, which is 

 intended to indicate the differences in time of reaction for the dif- 

 ferent senses gives these averages : Visual reaction-time .185 sec- 

 ond ; tactile, .148 ; auditory, .139. Not even the mean variability 

 is given in connection with the averages. Since reaction-time 

 varies with the strength of the stimulus it is possible by varying 

 the stimulus-intensity to get any one of the above reaction-times 

 with any of the qualities of stimulus named. This being true, 

 how are we to make valuable comparisons of reaction-times to 

 different kinds of stimuli ? 



As before stated the threshold intensities of all modes of stimu- 

 lation may be regarded as directly comparable. No matter what 



1 Jastrow, Joseph : "The Time Relations of Mental Phenomena," New York, 1890, 

 p. II. 



