4 MASS. EXPERIMENT STATION BULLETIN 414 



laboratories, have been trained to make tests without knowing what they mean. 

 Such technicians work by a rule-of-thumb method and have no alternative but 

 to interpret results as directed in the pages of a book. 



It is also unfortunate that records of tests, particularly of water and milk 

 supplies, are frequently made available to the public. Few lay people have had 

 the requisite training to enable them to interpret results of tests, and they do 

 not know the background of the materials being tested. In the case of milk 

 tests, one should never judge a supply on the basis of one or two tests. Too many 

 things can happen to a .sample of milk after it leaves the dairy to make the result 

 of a few tests conclusive. A supply should be considered unsatisfactory only 

 after check tests have been done, and after inspections have been made all the 

 way from the cow to the laboratory to determine the source of trouble. A water 

 test can be evaluated properly only if one has information regarding the source 

 of the water, and this is best obtained by a sanitary survey of the source. 



In the discussion of the experiments presented here, the author has no intent 

 or desire to find fault with any "Standard Method." No trained scientist, and 

 this includes those responsible for setting up the methods, would claim infalli- 

 bility for any test proposed. Tests must be kept simple and easily made if they 

 are to be useful in testing samples frequently and in large numbers. The intent 

 is that tests should be only general indicators of the quality of the materials 

 tested. Results should be interpreted with judgment and discretion. It has 

 been said that in medical diagnosis a laboratory test should be considered as 

 equivalent to one symptom only. A similar value should be placed on bacterio- 

 logical tests included in the various "Standard Methods." All of the "Standard 

 Methods" procedures are being scrutinized constantly by their sponsors to keep 

 them up to date. Promising new methods are investigated and some of them 

 are included in each new edition. 



Although it is not specifically stated in the publication, the "Standard Methods 

 of Water Analysis" have been developed particularly for the testing of public 

 water supplies. The main reason for this is that professional people in the fields 

 of public health and water works engineering are the ones primarily concerned 

 in promoting the development of analytical and testing methods for water, and 

 public water supplies are naturally their main interest. 



Public water supplies are usually chlorinated at least, and in many instances 

 chlorination is preceded by sedimentation and filtration. Logically, such water 

 is expected to be free from all types of bacteria that would give positive laboratory 

 tests, even though some of these types might have little immediate sanitary 

 significance. The standard of interpretation is desirably severe. It often hap- 

 pens, however, that raw water from wells and springs may give positive tests of 

 doubtful sanitary significance, and a sensible interpretation of results becomes a 

 difficult problem. 



In the eighth edition of the "Standard Methods of Water Analysis," this 

 statement appears: "At the same time, it is recognized that any one book cannot 

 possibly meet all of the needs of those engaged in laboratory work in this field 

 and that special methods must be developed to meet local conditions. ... It is 

 realized that this edition must represent only another milestone in progress . . ." 

 In the spirit of that statement, it was the purpose of the experiments reported 

 in this bulletin to attempt to find a basis for the interpretation of the "Standard 

 Methods" when employed for the testing of raw waters from farms and rural 

 homes, and to carry out certain studies that would permit a better understanding 

 of the bacteria responsible for positive tests of uncertain significance. 



