THE MICROBIOLOGY OF THE ATMOSPHERE 



requires sixty-nine infections (that is, 460 minus 391 according to the 

 table). 



TABLE XXVI 



With acknowledgements to Drs. S. B. Fracker and H. A. Brischle of the United States 

 Department of Agriculture. 



If the distribution of infections is at random among the hosts, a 

 straight hne will be obtained when the logarithm of the percentage that 

 remain uninfected (100 minus the percentage infected) is plotted against 

 the number of infections. The slope of the line is given by 'b' = — 0-00434. 

 Blackman (1942), in ecological studies of flowering-plant communities, 

 found that the occurrence of plants on quadrats may depart from the 

 expected random distribution and yet still give a reasonably straight line 

 when plotted as before. The slope of the observed line, however, differs 

 from that of the expected line. Blackman's 'correction factor', the ratio 

 of the slope of the expected line to the slope of the observed line 

 (K = bexpccted/boLservcd), thcn givcs a uscful measure of the departure from 

 the random arrangement. 



Various other mathematically plausible formulations of this deviation 

 have been attempted and are reviewed by Fracker & Brischle (1944). 

 Non-random distribution may result from various factors such as repulsion 



164 



