这句话有两个理解，但either way这个TFR都不是一个合理的统计，因为它忽略了是人们推迟结婚生BB的时间，而不是forsake the whole deal。第一个理解是假设每个cohort的在cohort时间内生育的weighted average。比如cohort是20-25，25-30，30-35，for simplification 35岁以上不能生，以前第一次统计人们在25岁前都起码生了一个，生育率为1以上，30岁前要生的都生完了。现在5年后的统计20－25岁（5年前是15 -20岁）那些人还是一生要生2个，但都推迟了5年。那么这年20-25岁生育率为0，25－30岁（5年前的20-25的人）的是少于1大于0，30－ 35岁（5年前25-30的人）是0（因为都生完了）。那么生育率当然是少，但忽略了现在20-25岁的人到了30-35岁时的生育率不会是零。
另一个理解是假设这个cohort的人现在的生育率是她5年10年到不能生为止的生育率。这个的谬误就更加明显了。
合不合理查一查就知道了 来自wiki
The total fertility rate (TFR, sometimes also called the fertility rate, period total fertility rate (PTFR) or total period fertility rate (TPFR)) of a population is the average number of children that would be born to a woman over her lifetime if she were to experience the exact current age-specific fertility rates (ASFRs) through her lifetime. It is obtained by summing the age-specific rates for a given time-point.
Graph of Total Fertility Rates vs. GDP per capita of the corresponding country, 2004. Only countries with over 5 Million population were plotted to reduce outlyers. Sources: CIA World Fact Book
Graph of Total Fertility Rates vs. GDP per capita of the corresponding country, 2004. Only countries with over 5 Million population were plotted to reduce outlyers. Sources: CIA World Fact Book
The TFR is a synthetic rate, not something that is actually counted. It is not based on the fertility of any real group of women, since this would involve waiting until they had completed childbearing. Nor is it based on counting up the total number of children actually born over their lifetime, but instead is based on the age-specific fertility rates of women in their "child-bearing years," which in conventional international statistical usage is ages 15-44 or 15-49.
The TFR is therefore a measure of the fertility of an imaginary woman who passes through her reproductive life subject to all the age-specific fertility rates for ages 15-49 that were recorded for a given population in a given year. The TFR represents the average number of children a woman would have were she to fast-forward through all her childbearing years in a single year, under all the age-specific fertility rates for that year. In other words, this rate is the number of children a woman would have if she was subject to prevailing fertility rates at all ages from a single given year, and survives throughout all her childbearing years.