APPENDIX n. 305 



The coefficient of correlation may be very small and yet 

 correlation may be a certainty if the coefficient is more than 

 six times the probable error. 



In the case we have been considering, of the relation 

 between supply and price, the coefficient was - -440 (i.e., 

 inverse correlation), but the probable error was only -079. 

 As this is less than one-sixth of the coefficient, correlation 

 may be said to exist. In the case of wheat yield and rainfall 

 for Canterbury, correlation was not established, for the co- 

 efficient was only + -250 and the probable error !. As was 

 explained in Chapter IV.* this was due to the fact that the 

 rainfall may vary considerably from the average without 

 greatly affecting the yield. Thus movements in the yield must 

 have been due partly to other causes. This is shown by the 

 relatively high probable error. 



Further information upon the whole problem of correlation 

 may be sought in 



King : Elements of Statistical Method. 



Bowley : Elements of Statistics. 



Elderton : Primer of Statistics. 



*See page 67. 



