DIFFICULTIES 



diplomacy or tact. It is not clear that Semmelweis' efforts had 

 much, or indeed any, influence on the final acceptance of the 

 principles he discovered. Others seem to have solved the problem 

 independently.^* 



Errors of interpretation 



For want of a more appropriate place, I shall mention here 

 some of the commoner pitfalls which are encountered in inter- 

 preting observations or experimental results and which have 

 not already been discussed. 



The most notorious source of fallacy is probably post hoc, 

 ergo propter hoc, that is, to attribute a causal relationship 

 between what has been done and what follows, especially to 

 conclude in the absence of controls that the outcome has been 

 influenced by some interference. All our actions and reason 

 are based on the legitimate assumption that all events have their 

 cause in what has gone before, but error often arises when we 

 attribute a causal role to a particular preceding event or inter- 

 ference on our part which in reality had no influence on the 

 outcome observed. The faith which the lay public has in 

 medicines is due in a large measure to this fallacy. Until very 

 recently the majority of medicines were of negligible value and 

 had little or no influence on the course of the illness for which 

 they were taken, nevertheless, many people firmly believed when 

 they recovered that the medicine had cured them. A lot of people 

 including some doctors, are convinced that certain bacterial 

 vaccines prevent the common cold, because by a fortunate coinci- 

 dence some patients had no cold the year following vaccination. 

 Yet all the many controlled experiments done with similar 

 vaccines failed to show the least benefit. The controlled experi- 

 ment is the only way of avoiding this type of fallacy. 



Much the same logical fallacy is involved in wrongly assuming 

 that when an association between two events is demonstrated, 

 the relationship is necessarily one of cause and effect. Sometimes 

 data are collected which show that the incidence of a certain 

 disease in a quarter of a city which is very smoky, or which 

 is very low-lying, is much higher than in other quarters. The 

 author may conclude that the smoke or low-lying ground pre- 

 disposes to the disease. Often such conclusions are quite 



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