20 



ROBERT M. YERKES 



It was my aim so far as possible to present to a given subject 

 each day the ten settings under a given problem in order, with- 

 out interruption. If for any reason the series of observations 

 had to be interrupted, it was resumed at the same point subse- 

 quently. Occasionally it was found desirable or necessary to 

 present only five of the series of ten settings in succession and 

 then to interrupt observations for an interval of a few minutes 

 or even several hours. But as a rule it was possible to present 

 the series of ten settings. All things being considered, it proved 

 more satisfactory to give only ten trials a day to each subject. 

 Frequently twenty and rarely thirty trials were given on the 

 same day. In such cases the series of settings was simply re- 

 peated. The only pause between trials was that necessary for 

 resetting the entrance doors and replenishing the food which 

 served as a reward for success. 



