180 



BIOLOGY OF DEATH 



The underlying idea was to get the partial or net 

 correlation between x 1 and x 2 , while x and x 3 are held 

 constant. If the mortality of infancy is selective, its 

 amount should be negatively correlated to a significant 

 degree with the mortality of the next eight years when 

 the births in each district considered are made con- 

 stant and when the general health environment is made 

 constant. Under the constant conditions specified a 

 negative correlation denotes that the heavier the infan- 



TABLE 22 



Snow's results on selective death rate in man. English and Prussian 



rural districts 



tile death rate in a cohort of births the lighter will be 

 the death rate of later years, and vice versa. The last 

 variable, x 3 , is the one chosen, after careful consideration 

 and many trials, to measure variation in the health envi- 

 ronment. If any year is a particularly unhealthy one an 

 epidemic year for example then this unhealthiness 

 should be accurately reflected in the deaths of those mem- 

 bers of the population not included in the cohort 

 under review. 



