Oportunidad de mejora para mitigar los impactos

Brass (1969, ibid) devised this method and applied it to the 1967 data from this survey. The opportunity is taken here to apply it to all subsequent years of survey data, to study the value of the data and the method as well as of the results. The following description of the method is based on that given in his paper.

The principle is as follows: if the data have been standardised to a population with equal numbers of women in each five year age group of the reproductive period, the ratio of all:first births in the year is equal to the total fertility rate (TFR) divided by the proportion of women who become mothers at the current rates (Fx ) . If the rates remain constant, this ratio also corresponds to the mean completed family size of mothers (Fm), i.e. -

Standardised All Births TFR Standardised First Births F^

The proportion of women who bear at least one child in the reproductive period (F^) can usually be estimated relatively accurately (probably from census data) and its use as a multiplier of the ratio of all:first births gives the total fertility rate.

However, the disadvantage of the method is that it is heavily dependent on the number of first births recorded and these are

particularly vulnerable to differential reporting (see Chapter 3). The information on the full birth order obtained in the birth notification scheme can be utilised to counter this problem. If a suitable

curve can be fitted to the numbers by birth order, more reasonable values for the proportion o f first births can be estimated from it. Although this can be done theoretically, Brass has devised a simpler approach, more in keeping with the nature and quality of the data, whereby the proportions of current births of each birth order are compared with the proportions of the corresponding birth orders of a reference birth distribution.

To do this, several stages of preliminary calculations are required:-

1) The mid-year populations of all women by five year age groups between IS and 50 years in the Solomon Islands for each year of the survey data (see Table 3(i)) are calculated using the 1959 and 1970 Censuses. The details of the method can be found in Appendix 3.

2) The factors necessary to raise the numbers of women in older age groups to the numbers in the age group 15-19 are calculated from these and the appropriate factors then applied to the number of births by order for each age group o f mother for each year of data (see Table A.l) to obtain the number of current births standardised by age of women

(i.e. where the numbers of women in each five year age group are the same as for the 15-19 years group). The birth performance in a single year by a cross-section of women can now be interpreted as that of an

'artificial' cohort over the reproductive period.

3) The births by order are then summed across all ages of the reproductive period to obtain figures for the number of women who will have had 1, 2, 3, etc., children by the end of childbearing. It

woman.

4) If these total figures are expressed as proportions of the total adjusted births born to this 'artificial' cohort based on women aged 15-19, the resulting values represent the proportions of women who have had births of the given order and above (see Table 3(ii)(b)). These are the adjusted observed proportions of births which need to be compared with a reference distribution of births by order.

5) The construction of such a reference distribution is done from the reports of women aged 40-49 (i.e. at the end of their childbearing period) at the 1959 and 1970 Censuses of the total number of children born alive to them. Cumulation gives the numbers of women with one or more, two or more, etc., children (i.e. the number of children of first birth order, second birth order, etc.). Division by the total number of children gives the proportion of all births which are first order, second order, etc. In a manner analagous to the derivation of the proportions of women in each age group in each year (see Appendix 3), the proportions of births of each birth order to women aged 40-49 years are calculated for each year of the survey and these form the terms of the empirical reference distribution (see Table 3(ii)(a)).

The standardised birth order distributions are plotted against the corresponding empirical reference distributions for each year of

survey data (see Figures 7 and 8 for sample graphs from two years' data). Regression lines were calculated and drawn on each graph. These were based only on parities 4-10 inclusive, as parities 1-3 were under study potentially and parities 11-13 were based on very small numbers and so both groups were omitted.

It can be seen that the point for first order births (and to a lesser extent the points for 2nd and 3rd order births too) shows a large discrepancy from the consistent trend shown by births of higher orders. There is no basic reason why this trend should be a straight line. A slight curvature would, however, make little difference to the estimating of values for 1st, 2nd and 3rd order births which are the

F I G U R E 7 C O M P A R I S O N OF B I R T H O R D E R D I S T R I B U T I O N W I T H R E F E R E N C E D I S T R I B U T I O N IN 1968 KEY: x b i r t h o r d e r s 0 a d ju s t e d f i r s t b ir t h s ---- r e g r e s s io n l in e

F I G U R E 8 C O M P A R I S O N OF B I R T H O R D E R D I S T R I B U T I O N W I T H R E F E R E N C E D I S T R I B U T I O N IN 19 7 5 KEY: x b ir t h o r d e r s q a d ju s t e d f i r s t b ir t h s ---r e g r e s s io n lin e

F I G U R E 8 COMPARISON OF BIRTH ORDER D I S T R IB U T IO N