c - usaid

10
Regarding the fourth index C , Bongearts and Potter have
explained why it can bo gonora lLy assumed to be age-invariant (at
any given point of time, because over time it is slowly increasing .
"Although postpartum infecundobi tity in reality rises
sLightly with ago, the other components of birth intervals
increasa also, so that tha fertility-inhibiting impact of
postpartum infecundabi Lity changes very Little with age. It is,
therefore, simpler and more convenient to use the same equation
for' C in all age-g roups end to assume the duration of postpartum
infacundahi lity to be ago invariant". /2
Tho aggregate ferti Lity model is based on an extension (over
time) of the central equation (2.1)
so as to include the base year
(0) and the target year (t), and by considaring the following
ratio:
x C ( t) x C (t)
-. - - - -
x C I (t) x TF(t)
TFR(t) C m (t) c - - -
a - - - - - - - - - . (2.*6)
TFR[O) C m (0) x C c (0) x C a (0) x C i (0) x TF(O)
Si nce our mai n inte r st is in C , its effect can be isolated
c
by combining thc ratios of the other indices Into a single factor
F( t), which moans that
C (t) . C (t) . C (t) . TF (t)
FCt ] ------ - --------------- ---. . . (2..[.7
C a [(O) . C ai (r) . C (Co) . TF (0]
Assuming that all other terms ore known except u(t] which is
CPR -­
100
., then -.. a simple re-arrangement of terms leads to the formula- ­
1 TFR~t) 1
uft) -- - .[ x-------------x (1-1.08B u[O} x
e(O)} ]... [2.8]
1.08xe(t)
TFR(O) F(t)
------------------------------------------------------------------------
/2 Bongaarts, J. and Potter, R. G., Fertility, Biology, and
Behavior, An AnaLysis of the Prox.imate Determinants, Academic
1 1
Press, Now York, 1983, pp. 6- 7