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I'm having trouble putting this into plain English.

According to the UN World Population Prospects glossary it means:

The mean age at childbearing is the mean age of mothers at the birth of their children if women were subject throughout their lives to the age-specific fertility rates observed in a given year.

The "their children" part trips me up.

Can I understand this measure as "average age of entry into child bearing" or "Average age of first child"?

Tomulent
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    Do you not still count as a child even if you were not the first one born to a particular mother? – whuber Dec 28 '17 at 19:44
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    Sounds very confusing but I think it is off topic for this site. – Michael R. Chernick Dec 28 '17 at 19:44
  • Any idea what community this could find a home in? @MichaelChernick – Tomulent Dec 28 '17 at 20:09
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    I'd say it is statistics and therefore on-topic for this site. Maybe it is not the mathematical stuff that you more often see here and also it is more about definitions rather than purely statistical issues (although statistics may be considered to start with definitions or at least be intertwined with it). – Sextus Empiricus Dec 28 '17 at 23:08
  • @Tomulent perhaps post this on English.stackexchange.com? They are wizards at interpreting the gobbledygook that we scientists conjure up. – AdamO Dec 28 '17 at 23:39

2 Answers2

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It is the mean age at childbearing (any child, not just the first), calculated, for a specific year based on the age specific fertility rates.

I guess that this is done in this way, in order to correct for variations of cohort sizes (population composition).

Changes in certain statistics may occur just because people are getting older or due to other changes in correlations composition and not so much due to certain changes in causal effects.

Consider for instance the following problem with analogous issues of compositions changes: many old people have diabetes, a higher BMI, and lots of problems with cardiovascular diseases. Then we see in populations that are getting older an increase of these health problems, not necessarily (solely) due to causal effects like bad habits, but just because people are getting older.

(although this is a bad example, in the sense that, even if you correct for age and gender those health problems are often still increasing, still in some countries the tide is changing for some problems which means they decrease if you correct for age and certainly in most cases the effects are much smaller than what it might seem or some people try to make you believe)

Say you use these data for Egypt 1997-2000 based on age categories per five years

age    fertility  births
15-19  0.051      764
20-24  0.196      2304
25-29  0.208      1994
30-34  0.147      1295
35-39  0.075      564
40-44  0.024      161
45-49  0.004      19

Then you get

$$\frac{17 \cdot 51+22 \cdot 196+27 \cdot 208+32 \cdot 147+37 \cdot 75+42 \cdot 24+47 \cdot 4}{51+196+208+147+75+24+4}=27.6$$

which differs from the situation when you compute the mean based on the number of births:

$$\frac{17 \cdot 764+22 \cdot 2304+27 \cdot 1994+32 \cdot 1295+37 \cdot 564+42 \cdot 161+47 \cdot 19}{764+2304+1994+1295+564+161+19}=26.4$$

Note that with this way of calculating a mean age of childbearing, the number is not influenced by differences in the age composition of the population (for instance if there is relatively less teenagers and more women in the forties then the mean age of childbirth might go up) and represents more precisely a difference in habits (for instance an increase because relatively less teenagers and more women in the 40-ies are getting babies)

Sextus Empiricus
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  • Statistics of first born babies vs total born babies are very interesting (e.g. we may see opposite trends in numbers), but it is not the case considered with this statistic here. – Sextus Empiricus Dec 28 '17 at 23:10
  • Is another way to say that, "The average childbearing woman in 20xx was X years of age"? – Tomulent Dec 29 '17 at 02:12
  • Indeed, you could say that, just 'another way'. But you have to realize that a mean/average can be computed in many different ways, is ambiguous, and therefore that definition is so verbose (in order to be more precise). Note that my answer is different to the other answer by accumulation. – Sextus Empiricus Dec 29 '17 at 08:39
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My interpretation is that this is calculated by taking each age, finding the number of children borne by women at that age, multiplying that number by the age, summing the results, and then dividing by the total number of births. It should be the same as the average, over all children born that year, of the children's mother's age. That is, if you record for each child born that year the age of their mother, and take the average of those numbers, that should be the same as this statistic.

Acccumulation
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