Summary and Conclusions 137 



be adopted to establish the existence of the cycles and 

 to ascertain their characteristic lengths, amplitudes and 

 phases. In our investigation, the choice of rainfall data 

 was suggested by the scope of our general problem. 

 Supposing that we could find definite periods in the 

 varying amount of the rainfall, we should then desire to 

 know the relation of rainfall to the yield of the crops, 

 and the relation of the yield of the crops to Economic 

 Cycles. It was necessary, therefore, that the data 

 of rainfall should refer to an area in which important 

 crops are produced, and it was desirable that the data of 

 both rainfall and crops should refer to a highly dynamic 

 society. For these reasons we collected the material 

 for our investigation from the central part of the United 

 States. 



The method adopted in an investigation of the 

 periodicity of rainfall must satisfy three conditions: 

 (1) It must exhaust the data in the search for possible 

 cycles; that is to say, the data must be made to yield 

 all the truth they contain relating to the particular 

 problem in hand. Frequently in the past, spurious 

 periodicities have been presented as real periodicities, 

 chiefly because the investigator started with a bias in 

 favor of a particular period and did not pursue his 

 researches sufficiently far to determine whether his 

 result was not one among many spurious, chance 

 periodicities contained in his material. In the search for 

 real periodicities the data must be exhaustively ana- 

 lyzed. (2) The method must render possible the dis- 

 crimination between a true periodicity, having its 



