330 ROBERT S. BREED AND W. D. DOTTERRER 



and 7 for all groups of plates having more than 20 and less 

 than 400 colonies per plate, the worst showing being made by 

 the plates having 20 to 30 colonies per plate and the best by 

 the plates having 100 to 200 colonies per plate. 



4. The discrepancies which occurred in counts made from 

 plates having less than 50 colonies per plate were more fre- 

 quently caused by too many colonies on the plates than by too 

 few colonies. This excess is undoubtedly due to the influence 

 of chance air contaminations which took place during the plating. 

 Where the plates have a small number of colonies on them a 

 few extra colonies of this sort produce relatively wide discrep- 

 ancies. 



5. The discrepancies in counts made from plates having more 

 than 50 colonies per plate were more frequently caused by hav- 

 ing too few rather than too many colonies on the plates. The 

 frequency of this type of discrepancy became very marked 

 where the number of colonies exceeded 200 per plate. The 

 probable explanation of the excess of this type of irregularity 

 is that of overcrowding. Since however there was always a 

 certain percentage of discrepancies caused by having too many 

 colonies on the plate even where there were more than 400 colo- 

 nies per plate, it is evident that not all of the irregularities are 

 caused in this way. 



6. Counts made from 20 dupUcate samples of the same milk 

 in five series of analyses showed 27 out of a possible 100 wide 

 discrepancies in the counts obtained from an average of two 

 plates made from a 1: 1000 dilution. The number of colonies 

 of these plates averaged more than 0.5 and less than 16.5 for 

 the two plates, with one exception where the average was 44. 

 Counts made from the 100 pairs of 1: 100 plates which had 

 more than 24 and less than 125 colonies as the average of the 

 two plates, showed only 3 out of a possible 100 wide discrepancies. 



