Population Projections - DefinitionEmpirically based calculations of future or past population numbers under specified assumptions about changes in population growth and its components.

From census bureau -- “Projections are estimates of the population for future dates. They illustrate plausible courses of future population change based on assumptions about future births, deaths, international migration, and domestic migration.”

Population projections and estimates 1. Projections - uses

i. refer to dates for which there are no observed data ii. Basis for decision making/policy

iii. Basis for understanding components of change/implications of change

2. Estimates - uses

i. Basis for federal funding allocations ii. generating denominator for rates

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Population Projections 1. Estimates of future population size, age structure

a. Requires assumptions about mortality,

fertility, migration

2. Shorter-term estimates

a. Depend less on assumptions because people

are already born

b. Relatively fixed characteristics vs. relatively

variable characteristics 1

Value of short-term projections

1. Depict likely changes in group composition

(e.g., elderly, near-elderly, others)

a. Identify, quantify potential policy issues

2. Understand process of cohort replacement

a. Effects of cohort size and characteristics

2

Projections of group characteristics based on one census - very simple method

1. Forward survive population by age (and sex) a. Use life table survival ratios

2. Assume constant mortality (unchanging SR) 3. Assume homogenous mortality

a. No cross-group differences in survival ratios b. Group-specific SR may be available

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Example – Racial composition of US pop age 65+

1. Inputs

a. Census tabulation of race by age (and sex)

b. Life table for corresponding year

2. Outputs

a. Projected population 10*x years into future

(by age)

b. Projected population 10*x years into future

(by age and race) 4

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Data – From 2000

Age 2000 Pop 2000 White pop 2000 % White Life table

survival ratios Under 5 19175798 12859892 0.67 5 to 9 20549505 13944882 0.68 10 to 14 20528072 14322638 0.70 0.9979 15 to 19 20219890 14167148 0.70 0.9970 20 to 24 18964001 13064891 0.69 0.9936 25 to 29 19381336 13501773 0.70 0.9909 30 to 34 20510388 14818786 0.72 0.9899 35 to 39 22706664 17031493 0.75 0.9880 40 to 44 22441863 17265995 0.77 0.9835 45 to 49 20092404 15810626 0.79 0.9758 50 to 54 17585548 14213875 0.81 0.9642 55 to 59 13469237 11107247 0.82 0.9471 60 to 64 10805447 8945842 0.83 0.9198 65 to 69 9533545 8040225 0.84 0.8784 70 to 74 8857441 7648193 0.86 0.8195 75 to 79 7415813 6530019 0.88 0.7367 80 to 84 4945367 4408597 0.89 0.6202 85 and over 4239587 3778504 0.89 0.3046 Total 281421906 211460626 0.75

Steps - 1

1. Forward survive 2000 population by age L

a. x5

10x5200010-x5

10x5

200010-x5

*2010x5 L

*PS*PP +==

b. x5

10x5*201010-x5

10x5

*201010-x5

*2020x5 L

L*PS*PP +==

c. Remember to use appropriate procedure for open ended interval

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Steps – 2

2. Multiply projected population by corresponding racial composition measures

Wa. 2000

10-x5

200010-x5*2010

x5*2010

x5 P*PW =

b. Calculate age specific population white at

x*10 years into future 3. Sum across age ranges of interest, divide by total

pop for corresponding age group

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Results

∑∑+

=

+

=∞ ÷=

85

65

tx5

85

65

tx5

t65 PW%W

xx

Year

2010 2020 2030 2040% 65+ White 0.84 0.81 0.78 0.74

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Alternative methods of projection (and estimates)

1. Growth rate projections

2. Balancing equation (component method)

3. Cohort component method

a. Use info on B, D, I, and E

b. Work with 5-year age groups

c. Project in 5-year steps

d. Separate projections by sex

4. Structural models 3

Projection variants

1. Middle

2. High

3. Low

4. Assume no change

2

Diagram of cohort component method

Launch-year population

Survived population

Childbearing age population

Survived births

Projected population

Fertility rates & infant/child survival rates

Survival rates

1

Diagram of cohort component method

4

Launch-year population

Projected migration

Survived population

Childbearing age population

Survived births

Projected population

Fertility rates & infant/child survival rates

Survival rates

Migration rates

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Level of projection

1. Easier at the national level

2. Migration is particularly problematic at the local

level

3. Important to incorporate external information at

the local level

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