82 MODES OF RESEARCH IN GENETICS 



that group and that instance the mean is perfectly 

 exact and precise to that degree of precision de- 

 noted by the unit of measure used, assuming that 

 no arithmetical mistakes have been made in its 

 computation. Thus suppose one measures the 

 stature of three men to the nearest inch, and then 

 calculates the average. The result is, without any 

 probable error, the average height, at the particu- 

 lar moment when they were measured, of those 

 three men ^ exact to the unit of measurement used. 

 It describes and measures precisely an attribute 

 f those men considered as a group. But if we 

 were to consider this result from the viewpoint 

 of whether IJPgave a reasonable measure of the 

 average height of men in general, or from the 

 viewpoint of whether it gave a proper value for 

 the mean height of these men when repeatedly 

 measured under varying conditions, it would clearly 

 be subject to a large probable error. It would, 

 in point of fact, have lost its character of precise 

 and definite knowledge, and have become a more 

 or less poor approximation. 



3. Precise knowledge of the degree of association 

 or contingency between different events or char- 

 acters within a group. This is furnished by the 

 method of correlation in one or another of its vari- 

 ous forms. By this general method we are able to 

 measure precisely the degree of resemblance be- 

 tween the individuals composing a group in respect 

 to one or more characters. So long as attention 



