JOHN S. FUI/TON, M. D. 145 



otherwise the curves, in their flatter portions, would have been 

 invisible. On a chart twice as long the mortality mode of scarlet 

 fever could have been shown, and the morbidity mode in three 

 times the length. 



Between the ages of 24 months and 20 years, practically all of 

 the mortality of childhood is included. Its magnitude compared with 

 that of the preceding two years, is as 70 to 154, or, taking the 

 difference of time into account, as 5 is to 95. The bulk of child- 

 hood mortality is compressed into the first two of eighteen years, 

 just as the bulk of infant mortality, in post-natal time is com- 

 pressed into the first three of twenty-four months. After the 

 mortality mode of childhood is passed, another distinctive mor- 

 tality mode is not reached until the age of 23 years, the centre of 

 the mortality curve for youth. Separate, by a very significant 

 number of years, from the mortality of youth on one side, and 

 by a significant number of months from the mortality of infancy 

 on the other, and unable to trespass seriously beyond the two- 

 year line or the 20-year line, this mortality of childhood seems 

 clearly distinguished from the adjacent mortality on either side. 



The validity of Pearson's distinction is further emphasized 

 by the occurrence of the morbidity mode later than the mortality 

 mode in this period. Elderton 2 has given to the sickness experi- 

 ence of the Friendly Societies of England and Wales the same 

 mathematical treatment which Pearson applied to mortality stat- 

 istics. This morbidity experience furnished no data for ages 

 under five years. Sickness values comparable with mortality 

 values are obtained by dividing the sickness rates (measured in 

 weeks) by 52.167 (52.167 weeks=l year), the quotient being the 

 number of individuals who, by carrying equal parts of all the 

 sickness observed during the year, would all be sick for a year, 

 and so pass out of the group exposed, just as if they died during 

 the year. 



Weeks of sickness ^ , ,..., - ,. . , , 



^ r~ I vx ^ -. Probability of being sick for a year. 



Number exposed X 52.167 



Elderton finds that the modal age of morbidity in childhood 

 occurs in the twelfth year, about nine years later than the mor- 

 tality mode. In youth the morbidity mode comes a year earlier, 

 and in middle age a little more than a year earlier, than the mor- 

 tality mode. The sickness mode of old age comes in the sixty- 

 third year, nine years before the mortality mode. 3 Beyond that, 



Graduation and Analysis of a Sickness Table ; W. Palin Elderton, Biometrika, 

 Vol. II, No. XV. 



Concerning the inverted order observed in childhood, Elderton says, "When 

 we get nearer birth it is not necessary for the origins of the sickness curves 

 to be earlier than those of the death curves, for, if the rate of mortality 

 is low (most obviously when it is decreasing), the fact of the sickness origin 

 coming after the deaths origin would only indicate that the incidence of sick- 

 ness in the particular period of life was such that the sickness rate was in- 

 creasing at a relatively greater rate than the death rates. The only thing 

 that seems necessary is that the sickness curve for any group should not start 

 later than the deaths curve." 



