Estimates of Birth Intervals in Pakistan, with and without the WFS Restrictions

Zubeda Khan, Ghulam Y. Soomro


Almost all the World Fertility Surveys (WFS), and those
repeating a similar pattern of pregnancy history data collection, like
the Population, Labour Force, and Migration Survey (PLM) carried out in
Pakistan in 1979-80, covered information on proximate determinants for
the last closed and open birth intervals. This paper, based on the PLM
data, discusses the methodological issue of data collection. The types
of restriction used have often been doubted as they have produced biased
estimates of contraceptive use and duration of breast-feeding, which are
important in the estimation of birth interval and in the ultimate
estimation of the structures of the relationship predicting fertility.
The representativeness of the last closed and open birth intervals is
limited if these are probed deeper in the time before survey, as the
proportion of births gets quite small. It is inappropriate to estimate
proximate determinants on fertility data that do not provide information
on all the births. Therefore, an attempt has been made to estimate
contraceptive use and breast-feeding with and without the WFS
restrictions. The logit model has been used with a dichotomous variable,
'whether the next live birth occurs or not', on proximate determinants
and other socio-economic variables to estimate the amount of biasedness.
The results reveal that there appears to be a higher proportion of
unbiased estimates if they are derived through the unrestricted sample;
and these results are in conformity with the results found elsewhere.
The biasedness of the restricted samples also affects the studies aimed
at observing the relationship between the predictor and the dependent

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