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Population pyramids - YouTube
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students welcome to another mr ness
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screencast
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please pause the video look these
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objectives over and get ready to copy
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some vocab and take some notes
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the first term is sex ratio this is the
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ratio of males to females in a
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population
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today you're going to learn how to read
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a special type of graph called a
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population pyramid
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and all population pyramids show sex
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ratios because they are really two
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separate graphs
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one for each sex
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take a look at this population pyramid
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of the united states
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i like to think of population pyramids
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as two bar graphs flipped on their side
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moving up and down the y axis are
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different age brackets usually in
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intervals of five
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on the x-axis
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we see the total number of people in the
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population that are in each age bracket
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so here we have two graphs
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one on each side on the left side we
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have the total number of males in each
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age bracket and on the right side we
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have the total number of females in each
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age bracket
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let's look at an example take this age
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bracket here which is 15 to 20 year olds
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on the x-axis we see on the left that
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there are about 11 million males and on
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the right
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there are about 11 million females
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so that means that in the year 2009 when
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this was made
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in the united states population
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there were 11 million 15 to 20 year old
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boys and 11 million 15 to 20 year old
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girls
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now i want you to practice by pausing
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the video
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and thinking about these questions
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in 2009 in the united states how many 80
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to 85 year old men were there
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how many 80 to 85 year old women were
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there
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pause the video and think about it
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if you said about three million of each
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you are correct
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if you got it wrong
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look back and study this more before you
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move on
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be careful sometimes the x-axis of a
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population pyramid shows the percentage
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of the population in each age bracket
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instead of the total number of people
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look at this one on the right
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this pyramid shows that in the 15 to 20
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year old age bracket
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males are three and a half percent and
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females are three and a half percent
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that means that three point five percent
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of the total u.s population in this year
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was 15 to 20 year old boys
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and 3.5 percent of the total population
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was 15 to 20 year old girls
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his population pyramids are giving the
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same information but expressed in a
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different way
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this one on the left
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shows the total population in each age
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bracket whereas this one on the right
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shows the percentage of the total
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population that is in each age bracket
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now let's look at some common population
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pyramid shapes that correspond with
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countries in each stage of the
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demographic transition
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as we look at these pyramid shapes i
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want you to focus on the bottom two
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thirds of the age brackets
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in every country no matter how developed
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people die of old age so these upper age
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brackets
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from about 75 and above will always have
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a similar shape
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it's the lower age brackets from 0 to 75
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that are important
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all right let's start with stage two
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the population pyramid of a stage two
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country is a pyramid with a wide base
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the overall shape is a triangle or
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pyramid but the widest age brackets are
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at the very bottom
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or base
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countries in stage two like niger have a
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lot more young people than old people
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there are a lot of baby boys and girls
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between the ages of zero and five
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but as we move up in brackets there are
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fewer and fewer people in each one that
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means that a lot of babies are being
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born but some of them die before they
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turn 5 and then some of those that make
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it to 5 die before they turn 10 and some
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of those who make it to 10 die before
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they've turned 15 etc
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anytime you see the bars moving inwards
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like this
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it means that people are dying
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in a country like niger the birth rates
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are high which is why the base is so
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wide
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but because health care is poor many
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children die at a young age and that's
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why the base slants inward
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like a pyramid
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now let's look at stage three
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the shape is a pyramid with a
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cylindrical base
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unlike niger where the base slanted
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inwards
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the base of a stage 3 country is mostly
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straight look at these lines they are
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straight up and down at the base there
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is no slant inwards until we get up
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until these middle ages
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that means that in a stage 3 country
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like pakistan
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babies don't die as they get older they
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survive
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we don't see any inward slant at the
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bottom
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remember that inward slant means people
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are dying and we don't see that until we
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get to about age 30.
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so
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unlike stage 2 where the base slanted in
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immediately
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in stage 3 we have straight sides at the
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bottom which i call a cylindrical base
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and as we get into the older age
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brackets we see a pyramid as the sides
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slant inwards
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stage four is a cylinder with a bulging
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middle
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at the bottom
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we have straight sides look at the very
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bottom
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but as we get up into our middle ages
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instead of slanting inwards
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the brackets slant outwards they bulge
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out that's why i call this a cylinder
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with a bulging middle
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and this outward slant means that as
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time passed women started to have fewer
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children
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look at this pyramid of the united
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states as stage 4 country and notice how
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between the
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15 i'm sorry between the 20 to 24
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bracket
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and the 25 to 29 bracket there is a
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bulge outwards
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that's because about 25 to 30 years ago
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women had more children than they did 20
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to 24 years ago
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so in the year 2000 which is about 20
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years ago women in america had fewer
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children than they had in 1995 which was
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five years before
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in other words
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between 1995 and 2000 the birth rate
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decreased
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remember always focus on the bottom two
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thirds of the pyramid
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the last stage is stage five and the
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shape is called a cylinder with an
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upside down pyramid base
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unlike stage 4 where the bottom h
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brackets had fairly straight edges and a
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stage 5 pyramid
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the bottom brackets are slanting
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outwards from the base like an upside
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down pyramid and then they have a
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straight sided cylinder in the middle
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ages
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this is what it looks like when birth
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rates continue falling and each
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generation is having fewer children than
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the previous one
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cylinder with an upside down pyramid
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base
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contrast that shape of stage 5 with the
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stage 4 shape cylinder with bulging
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middle
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in stage 4
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the bottom ears are straight but in
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stage five they start to slant outwards
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immediately
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and they continue to slant outwards as
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we move up in age
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the last term today is the dependency
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ratio
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this is the number of people not of
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working age divided by the number of
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people that are of working age
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so we have the total number of very
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young and very old people who can't work
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divided by the number of people who can
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work this number is important because
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working people pay the taxes that are
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needed to support the non-working people
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and we define working age as 15
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to 64.
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there are two types of dependency ratios
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the child dependency ratio and the old
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age dependency ratio
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child dependency ratio is the number of
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children under the age of 15 divided by
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the number of people between 15 and 64.
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the old age dependency ratio is the
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number of people 65 or older divided by
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the number of people between 15 and 64.
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a high child dependency ratio means that
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working age adults
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are having to work and pay taxes to
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support a lot of children
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a high old age dependency ratio
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means that working age adults are having
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to work and pay taxes to support a lot
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of elders
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both a high child dependency and a high
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old age dependency are bad for a
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country's economy
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because many people who can't work
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have to rely on the fewer numbers of
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workers
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countries in stages 2 and 3 have high
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youth dependency ratios high birth rates
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in those countries mean that there are a
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lot of children relative to working age
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adults
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think about niger where birth rates are
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so high
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on the other hand stage five countries
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have high elder dependency ratios
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because most people live to old age but
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few children are being born
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a good example of this is japan
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stage 4 is the sweet spot where both
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child and old age dependency is low
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this is the stage where total fertility
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rates are about 2.1 that magic number of
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replacement level
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a good example is the us
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all right here's some practice i want
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you to study this country's population
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pyramid and see if you can guess the
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following things take a look at a b c
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and d
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pause the video
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and
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try and answer them on your own
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and here are your answers how'd you do
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if you did well you are ready
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for tomorrow if not rewind the video and
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try again
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alright it's time to review the
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objectives again if you don't know the
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answer just rewind and review
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i'll see you in class
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bye students
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