450 



RESEARCH IN PROTOZOOLOGY 



an important point will be the determination of the reduction in 

 the variability of one measurement that arises on selecting in- 

 dividuals that are constant as to the other measurement. This will 

 be done by considering the standard deviations of one measure- 

 ment for individuals that are constant as to the other measure- 

 ment; for example, in the table at hand, we shall consider the 

 standard deviation in breadth of the 25 individuals that have the 

 length 12; similarly for the 32 individuals that have a length 13, 

 etc. Conversely, we shall consider the standard deviation in length 

 of the 50 individuals whose breadth is 7 and of the 32 individuals 

 whose breadth is 8, etc. A comparison of these standard devia- 

 tions within specific arrays of the table with the general standard 

 deviation of the variables under consideration is another function 

 of the study of correlation. 



The determination of the regression Hues and of the variability 

 within the arrays of a correlation table leads in the case of normal 

 correlation to an index number that is of considerable importance. 

 This index number is called the correlation coefficient and its 

 determination together with that of the other important elements 

 of a correlation table will be illustrated in the following ex- 

 ample : 



DETERMINATION OF THE CORRELATION BETWEEN THE LENGTH AND BREADTH OF 



TROPHOZOITES OF Giardta Bradypi 



Zfx _ I 



---^--^-0.5 



= 0.86 



fioy 



= 0.61 



= V//21/ = 0.7810 



