336 



Method of (Hfimatimj Bacterial Dcnaiti) 



index is or is not in accordance with the distribution predicted from 

 Elderton's tables, when 



and n' ^ n. 



The statistic x^ thus supplies an index of dispersion for sets of parallel 

 plates. If the bacterial counts conform to the Poisson distribution the 

 average value of )^ will be one less than the number of plates. For 

 sufficiently numerous sets of plates the agreement may be tested inore 

 exactly by the use of Elderton's Tables. 



6. The x^ Index of Dispersion applied to Cutler's Data 



The values of x^ obtained from the sets of four parallel plates, grouped 

 according to the value of the mean, are shown in Table VI. 



Table VI 



No obvious relationships are observable between the value of x^ and 

 that of ^;. There is indeed an excess of the exceptionally large values of 



