TABLE 1 presents the Monthly Live Birth-Rate in the City of Boston for the period January, 1900, through December, 1921.1 These data are presented graphically in Chart 1. It is easy to notice that the highest Monthly Live Birth-Rate occurred in December, 1907, when it reached 31.70. August, 1900, is very close to this peak month with a rate of 31.55. Next follows March, 1907, with its rate of 31.43. The lowest month during these twenty-two years was June, 1919, when the rate was 20.90 per 1000 population; close by is January, of the same year, with its rate of 21.40. There is, therefore, a difference of 1080 births per 100,000 of population between the peak month and the lowest month. In other words, the lowest month during this span of years represents a decline of 34.07 per cent (over one third) compared with the highest month.
A glance at Chart 1 shows that there has been a steady decline in the Monthly Live Birth-Rate during the past twenty-two years. The straight line running through the plotted rates is the trend line, or the line of "best fit," which has been secured by the Method of Least Squares. It is readily recognized that for this span of years a straight line gives a good "fit."2 The equation to the line____________________