Let us start with a simple question: What was the total fertility rate (TFR) for a given developing country in a given year? The total fertility rate—or the number of children, on average, a woman in a given population is expected to have in her lifetime—is a crucial indicator, as it, in conjunction with mortality and migration, determines the future size, growth rate and age structure of a population. It is a much more important and informative measure than the crude birth rate, which is simply a snapshot of the total number of births in a population divided by the size of the population in a given year. The crude birth rate can vary quite a bit from year to year, and is affected by the age distribution of the population—a younger population will usually have a higher crude birth rate, because there are more fertile women in the population, even if the number of children most women will actually bear is falling. By contrast, the total fertility rate is calculated by finding the birth rate for women at each age and totaling up the number of children, on average, a woman will have as she moves through her entire span of childbearing years, assuming she survives that entire span.
To calculate the TFR, one simply needs to have census or survey data that provides a count or estimate of the number of women at each age in the society, and whether or not they gave birth that year. So it is a straightforward exercise in counting women and births. Or so it would seem.
Unfortunately, there is nothing straightforward or simple about it. Even if one had a census that reached every household, people tend to be inaccurate in giving their age; women in particular tend to take a year or so off their true age, or round down to the nearest half-decade or round number. Moreover, while TFR should include all live births, women sometimes do not report births if the baby has died in the interval between the start of the year and the date of the survey or census. Add to this the normal problem of taking a census or doing a survey in less-developed societies: The less common that formal employment and permanent residence are within the society, the more difficulty there is in ensuring that a census or survey reaches or is representative of the entire population. So the basic numbers of women in each age group and their births are subject to miscounting.