## INTRODUCTION **XV**

# procedures for projecting fertility.

**_Data._** Base-period total fertility rates (TFRs) are derived from the same mix of sources as mortality rates, including _World Population Prospects 1988;_ official sources, as cited in _Population and Vital Statistics Report_ or in government publications; the U.S. Bureau of the Census (1985; Jamison, Johnson, and Engels 1987); and Monnier (1988). We also relied on various surveys, especially Demographic and Health Surv-vs, which were available for two dozen countries. Often, these sources give fertility estimates for a different period or in terms of the crude birth rate, and appropriate total fertility rates were obtained by projection of earlier estimates or approximated using available data on age-sex composition and age-specific fertility. The appendix lists the specific source for each country, and also indicates some cases where an informal Bank estimate was made in the absence of reliable data.

Future trends. Future fertility trends are specified by quinquepnial total fertility and by a year when the net reproduction rate reaches unity, i.e., when the average woman's fertility results in exactly replacing herself. (This is referred to here as the replacement year.) Separate procedures are applied for three stages of the fertility transition: the pretransition stage, when fertility is high and sustained fertility decline has riot started; the transition stage, when fertility is high to medium and sustained fertility decline is in progress; and the late-transition and posttransition stage. We discuss procedures for determining total fertility in each stage and then explain what age-specific patterns of fcrtility were applied.

Fot current purposes, a fertility transition is assumed to have started if a country has experienced a drop in total fertility of at least .5 points over any five-year period, or if total fertility is already below 4.5 after a more gradual decline (Bulatao and Elwan 1985). The pretransition stage is the stage before any such decline is evident. The late-transition and posttransition stage is defined by a total fertility rate one point above replacement level (about 3.15) or lower.

Countries in the pretransition stage are assumed to start a fertility transition in the quinquennium after combined male and female life expectancy reaches 50 years, but in no case later than 2005. The life expectancy threshold is slightly lower than the threshold of 53 years previously shown to be a universal precondition, though not a sufficient condition, for the start of fertility transition (Bulatao and Elwan 1985). Until a transition starts, total fertility is assumed to be constant. The one exception is where sterility is a significant factor, sterility is then assumed to decline linearly to 6 percent over three quinquennia, with each percentage point drop in sterility raising total fertility by .11 points (Frank 1983).

In the transition stage, the rate of fertility decline is based on the rate in the preceding quinquennium, assuming transition had already started in that period. With previous annual change represented by DTn, annual change during the transition is set at (-.05 + .5 DwF). However, limits are set on this change: it must be at least -.073 points and at most -.210 points, which may be taken to define slow and rapid fertility decline. (Previous analysis in Bulatao and Elwan 19S5 defined rapid decline as an annual total fertility change of -.2164 points, which is consistent.) If the formula does not apply because transition has just started, an average annual decline in total fertility of .102 points is imposed. Figure 3 shows the patterns of fertility decline implied by this average and by the slow and rapid limits. In the early part of the transition stage, the sterility adjustment is applied if appropriate to raise total fertility slightly.

In the late-transition and posttransition stage, fertility approaches replacement level, either from slightly above or slightly below it. Four alternative patterns, represented in Figire 4, are applied in this stage.