70 GENETICS AND EUGENICS 



(Q^ Q') so placed that between the two Hnes half the area of 

 the figure will be included. It is obvious that an individual 

 taken at random may fall in any part of the figure, but the 

 chances are even that it will fall inside or outside of the 

 probable error (Q, Q') since half the group occurs in each 

 position. The probable error of a determination of the mean 



equals ± 0.6745 —7=- Notice in accordance with this that 



the more individuals one observes the more accurate his con- 

 clusion, i. e., the less the probable error, but not in direct 

 proportion to the number observed but to its square root. 



The probable error of the standard deviation is expressed 

 by the equation, 



Ea =^ 0.6745 — ^ 



The probable error of the coefficient of variability is 

 expressed by 



CV 



Ec=^ 0.6745 



V^ 



n 



The probable error of the coefficient of correlation is 



expressed by 



r, ± 0.6745 (1 - r^) 

 £jr — 1= 



yjn 



The probable error of a determination of the cross-over 

 percentage between two linked characters is 



P (1 — P) . 



rt .6745\/ in which P is the observed cross-over 



T n 



percentage. (Haldane, 1919.) 



