far as possible to correct for them. His method may be described as follows:
The mortality of children born in any year, say 1903, in as large a series of districts as possible[1] is followed year by year. Working with this series which is homogeneous with respect to year of birth,[2] the problem to determine whether, allowing for environmental influence, the death rate of, e. g., the first year, or of the first two years, has any influence upon that of subsequent periods of life.
Now this factor, which for convenience we have designated as environmental,[3] is of great importance. The death rate differs widely from district to district, and in response to many factors.[4] Thus absolutely districts having a low mortality for the first year of life might have a low mortality for the second to the fifth years of life; districts having a high death rate for the first twelve months of life would also have a high death rate for the thirteenth to the sixtieth month of life, since many of the causes operating in the two cases (bringing about high or low death rates) affect both age periods.
Thus a high mortality of infancy does not necessarily imply a low mortality of childhood or a high military efficiency. This is true because any influence of selection is largely obscured by such factors as ethnic or social composition or physical environment in the various districts. Before one can say anything at all concerning the possible
- ↑ It is very important that the subdivisions be as numerous and as homogeneous within themselves as is consistent with data for trustworthy death rates. For, obviously, the death rate in one district in a given year may be abnormally high (or low) because of purely local and transitory conditions. These are precisely the factors which make for a high or low selective death rate. By lumping a number of districts together one may cancel out the very terms he is seeking to investigate! The value of some of the published work is nullified by the neglect of this point.
- ↑ Obvious as the importance of this point is, it has been entirely overlooked or disregarded by some. To determine that the death rate of children to 1 year of age and that of those 1 to 5 years of age in a series of districts are correlated for a given year, say 1905, proves nothing at all concerning a selective death rate. The infants exposed to conditions (in the various districts) resulting in high and low death rates for their first year of life in 1905 are being compared with those exposed to the action of selection under what may have been widely different conditions in 1904, 1903, 1902 and 1901. The whole factor of the variation in mortality from year to year due to epidemics, meteorological conditions, economic changes, etc., is thus left entirely out of account.
- ↑ Local influence might have been a better term, since racial composition as well as environment may play a part.
- ↑ The fact that the death rate is to so great extent within the control of the sanitary and charity boards is sufficient general evidence for this statement. A quantitative demonstration is seen in the fact that a correlation is found between, e. g., the birth rate and infantile mortality; also between artificial feeding rate and infantile mortality. See Greenwood and Bevan, Jour. Hyg., 12: 5-45, 1912.
