The method of calculation used to derive a proportion of women still fecund for individual years of age or for five-year age groups is illustrated in figure A8.1, which refers to this calculation for women in the age group 35–9. In the figure a part of the life history of eight women is shown by lines within the figure representing events in their lives between the ages of 34 and 41. A cross represents a birth, a bold cross a last birth, while the point at which fecundity ceases is indicated by a small vertical line 17.5 months later than a bold cross. The subsequent history of the woman is then shown as a broken line. To simplify matters a year is taken as 365 days and 17.5 months is taken as 533 days. The age of the individual at the birth of the last child, and again at the point at which fecundity ceases, is shown in years and days. In two cases the birth of the last child takes place after the age of 41 years, and in another case the last birth occurs during the 41st year. There is also one case in which, although the date of birth of the last child is less than 40 (38 years and 300 days), the age of the woman when her fecundity ceases is greater than 40 years.