**About the population pyramid and data**

** ** Population pyramids are commonly used by
demographers to illustrate age and sex structure of a country's population.
Countries that are expected to grow rapidly typically have a large
number of individuals in their reproductive years and the population
pyramid is wider of "heavier" at the bottom. In contrast, populations
that are expected to have slow, zero, or negative growth typically
have more individuals beyond reproductive age, and their pyramids
tend to be "top heavy". Population distributions of Mexico (rapid
growth) and Germany (negative growth) can be found in QELP datasets
#032
and #033
, respectively.

The US population pyramid is nearly symmetric about the vertical axis reflecting the
fact that all age groups contain roughly equal numbers of males and females. There are slightly more males
up until age group 20-25. The drop in the number of males as compared to females is quite substantial in
age group 25-29 (why?), and then after the number of females exceeding the number of males slowly grows. There is a population "bulge" in the middle of the pyramid representing the so called baby boomers of
ages 35 to 50, and another small bulge for ages between 10 and 20 years. The small bulge represents mostly
children of baby boomers and is an "echo" of the older and larger baby boom generation. The number
of people of working age (between 20 and 65) is about 162 million and the number older than 65 is 35 million; this
is roughly a ratio of 4.5 workers to 1 retiree. Will the same ratio exist in 15 to 30 years when the
baby boomers enter retirement age? What would the population pyramid look like if 4.5 workers were needed
to support every retiree in the future?

The number of individuals in one age group that move to the next age group is based upon
each age group's *survival rate*. For example, every five years a fraction of age group 15-19 will survive
to graduate to the 20 to 24 year age group. As a country ages, the population distribution and corresponding
pyramid shape will generally change. For example, the population bulge in the middle of the US pyramid will
(most likely) move upwards over time so that in 25 years the bulge will be present at the top of the pyramid.
Dynamic, time dependent population pyramids can be viewed at the extraordinary website http://www.census.gov/ipc/www/idbpyr.html
. This site is part of the much larger US Census International Data Base (IDB) website where additional country
by country demographics can be found.

*Source of the Data: US Census http://www.census.gov/*