Part 5:  Age Structure

 

The % of the Population 0 to 14, Over 65 and Over 80 Charts are easy ways of determining how "young" or "old" a population is. 

 

 

 
 

 

 

 
 

 

 

 
The Formulas for calculating dependency ratios are:
  • Youth Dependency Ratio =  Population Below Age 15/ Population Aged 15-64
  • Aged Dependency Ratio =  Population Aged 65 and over/ Population Aged 15-64
  • Total Dependency Ratio =  (Population Below Age 15 + Population Aged 65 and over)/ Population Aged 15-64

Dependency ratios can range in value from very small fractions (.03) to figures slightly above 1 (1.06).  Each ratio is a way of calculating how many dependent persons there are for each person of working age. The correct way of interpreting a Youth Dependency Ratio of .71 is to say that it indicates that for every person of working age there is about 7 tenths of a person below the age of 15.

 

 

 
 

 

 

 
A Total Dependency Ratio of 1.0 means that there is one dependent person for each person of working age.  Obviously, a "high" dependency ratio is more problematic for a population than a low dependency ratio. 

 

 

 
The Median Age Chart is a very nice "composite" chart that traces gross changes in the age structure.  It is an easy way to determine which population is "younger" or "older."

 

 

 
Try to "read" the age-sex pyramids of each population.   Focus on dramatic indentations or extrusions in the pyramid, the general shape of the pyramid, and changes in the shape of pyramid over time.  These you should attempt to explain by relating them to the fertility history of each population and, occasionally, to dramatic mortality events such as wars:
  • Can you "see" reflected in these pyramids any fertility and mortality trends that are evident in the fertility and mortality charts above?
  • Can you "see" reflected in these pyramids any significant historical events that might have impacted your populations.  In the 1950 pyramids can you "see" an impact of WWII or the Great Depression of the 1930s?  Can you "see" a "baby boom" or a "birth dearth" in the 1985 pyramids?  Can you "see" a recent fertility decline in your 2005 pyramids?
  • Scan each pyramid for significant differences in male/female composition as well.

 

 

 
An easy way aid to "seeing" a population's fertility history in its age-sex pyramid is to actually calculate the "year of birth" of each age group on a pyramid.  For example, in the 1950 age-sex pyramid the age group 0-4 was "born 1946-50" and the age group 5-9 was "born 1941-45" etc.  You can print out the pyramids and write these "years of birth" right on them.  That way if you see on your 1950 pyramid, for example, that the "age group 30-34" is exceptionally small you will know that this cohort is also the one "born 1915-19."  If you know that your population was a major participant in WWI, now you know that it likely experienced a sharp decline in births during that war -- and this explains the small size of the 30-34 year old cohort in the 1950 pyramid.

 

 

 
  • Be careful of "seeing" mortality declines in age-sex pyramid changes.  For instance, fertility has much more of an impact on the % of the population over 65  than mortality.  A high total fertility rate will produce a very low % of the population over 65 regardless of the level of mortality – and a low total fertility rate will produce a high % over 65.
  • High mortality caused by a war -- say that experienced by Russia during WWII -- often shows itself on an age-sex pyramid by there being many fewer males than females in the age groups that would have fought that war.  Males (i.e.., soldiers) were much more likely to be war-time casualties than females.  If there are similar numbers of males and females in "small" age groups then it is much more likely that this is a consequence of low fertility than high mortality.
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    Look at the year 2005 pyramid.  Look at the youngest cohorts.  What do you see?  Are the sides of these cohorts beginning to "flatten" or "tuck in"?   Hint:  if fertility were high and constant, then each cohort would be significantly larger than the one before it.  A flattening is evidence of fertility decline.  A check of the Total Fertility Rate Chart can confirm that this is the cause of the "flattening."