STATISTICAL METHODS IN PROTOZOOLOGY 



459 



TABLE 6 

 GROWTH OF ASEXUAL PARASITE 



If we plot these observations on ordinary arithmetic paper as 

 in Fig. 31, we note at once that an increase in size is not constant 

 for a given length of time but tends rather to be larger for the 

 later periods. If, however, we plot these observations on arith-log 

 paper as in Fig. 32, we see that the points tend to take a linear 

 position. This indicates that while the absolute rate of change 

 of these parasites is variable, the proportional rate of change is 

 practically constant. We may, therefore, fit to these points an equa- 

 tion of the type 



y = loa+^J! 



This may be done in several ways, one method being to take 

 the logs of the y's and relate these logs to their corresponding x 

 values by the method discussed under the head of correlation. 

 In this case, however, it will be sufficient to fit a line by the 

 method of inspection and the line shown in Figure 32 was drawn 

 in the position that seemed best to represent the points. After the 

 line is drawn, two values may be read from the line as follows : 



x= o (time 8 P.M.) 3; == 1.14 

 ;jr ^ 20 (time 4 p.m.) 3; = 20 



