Page 69 - IJPS-7-2
P. 69
International Journal of
Population Studies Age-adjusted measures for fertility transition
can be used in multivariable analysis fertility transitions, age group the mother belonged to at the time of giving
it was important to make sure that the data were of birth. This is illustrated in the figure below, supposing
acceptable level of quality. Consequently, we analyzed one is computing fertility rates using data from a survey
the quality of the age data for the 23 surveys used in this conducted in the year 2010. The figure also illustrates the
study using the Myer’s Index (MI). The findings from this point of difference between the ASMFR and CEB.
analysis are presented under results section and they show Figure 1 shows six births delivered from 2004 to
that the quality of the data from the four countries meets 2010 by three women (W1 – W3). For the calculation of
the minimum expected standards for this study. ASMFRs following the standard procedure recommended
2.4. Computation of fertility measures for DHS data analysis, only three of these births which are
highlighted in red would be considered. The numerator for
The fertility measures used in this study have different the ASMFR for the age group 15 – 19 will comprise one
computational demands. The CEB, because it measures birth by woman number two (W2) and four person-years
achieved fertility, is obtained by finding the average of exposure distributed as two for W2 and two for W3. For
number of children born to a defined birth cohort which the 20 – 24 age group, the numerator will be two births by
can be single-year or 5-year. The CEB measure is computed W1 and W2 who contribute three and one person-years
by dividing the total number of CEB by a cohort of women of exposure to the denominator, respectively. Using the
by the total number of women as indicated in the formula birth history data transformed into person-periods, the
below: ASMFRs are computed as:
C 45 −49
−+5
CEB nn = W nn (1) λ i α = exp ∑ bA ki + (2)
−+5
k
−+5
nn
k =20 −24
Using this measure, the average number of CEB to
the 45 – 49 age group, called completed family size, is the Where α is a constant term, b is the intercept, A is
ki
equivalent of the total fertility rate. The downside of this, dummy variables for 5-year age groups from 20 – 24 to
however, is that one must wait for 45 years to obtain the total 45 – 49 years with the 15 – 19-year age group being used as
fertility rate of a birth cohort aged 15 – 19 years. To obtain the reference category (Schoumaker, 2013). Using the same
a measure of the total fertility rate of a population without Poisson model, the TMFR is computed by multiplying the
exponentiated sum of the constant term and the regression
having to wait for 35 years, the age-adjusted measures, coefficients for the respective age groups by five as follows:
namely, TMFR and its widely cited equivalent, TFR, are
often computed. It should, however, be acknowledged 45 −49
α
that data on marital fertility may not be easily accessible, TMFR = ∑ exp α + 5* exp + b k (3)
especially in countries that do not collect DHS data and do k =20 −24
not make marital status data publicly available from their
censuses.
The estimation of the TMFR and its constituents
ASMFRs, equations [2] and [3], take a different approach
from that for CEB. When computing the TMFR and
ASMFRs using the DHS data, the recommended approach
is to use births occurring in the 3 years preceding the
year of survey data collection (Croft, et al., 2018). Despite
producing a TMFR which is considered a synthetic
measure, this approach provides a current picture of the age
patterns of fertility at any given time. Calculating ASMFRs
involve dividing the number of live births occurring to a
cohort during a specified period by the total number of
person-years of exposure for the women of the given age
cohort. Schoumaker (2013) developed a Stata programme,
tfr2, which computes fertility rates using this approach
by applying a Poisson regression model on birth history Figure 1. Lexis diagram illustrating birth history data for individual
data. This program first transforms birth history data into women for a DHS survey conducted in 201
a person-period table such that births are counted for the Note: Adapted from Schoumaker (2013).
Volume 7 Issue 2 (2021) 63 https://doi.org/10.36922/ijps.v7i2.354
