1. Population Projections
Preparation for Cohort-Component Projections 1

2. From Estimates to Projections
The discussion follows chapter 8 of the
Census Bureau’s Population Analysis with Microcomputers. The methods used for
preparing the inputs for projection
are discussed in more detail in chapter 8. 2 2

3. Cohort-Component Projections
Cohort component projections are based on a
simple accounting formula:
Population at a future date is equal to:
Population at the current date
+ births,
- deaths,
+/- net migration.

4. Cohort-Component Projections
are based on the simple accounting formula:
…Plus detail on AGE and SEX

5. From Estimates to Projections
In this part of the workshop we will review estimates produced using various methods and select those to include in our projections
Estimates provide the basis on which we build cohort component projections 5 5

6. From Estimates to Projections
Lets go over the process of preparing estimates which can serve as the basic inputs for cohort-component projection of population:
Smoothing the base population above age 5 or 10 (if deemed necessary)
Adjustment of the base population under age 5 or 10 for consistency with fertility and mortality
“Moving” the base population to midyear
Projecting fertility, mortality, and migration 6 6

7. Cohort-Component Method
A base population is determined that agrees with known demographic characteristics of the country.
Levels of mortality, fertility, and migration are determined for around the base year and projected to future years.
Then the base population is projected into the future according to the projected components of change. 7 7

8. Cohort-Component Method
Two aspects of mortality have to be projected:
the level and the pattern by age and sex.
Two aspects of fertility have to be projected:
the level and the age pattern.
Two aspects of international migration have to be projected:
the total number of migrants (either net or separate immigrants and emigrants) and the age and sex composition.
Alternatively, net migration rates by age and sex can be used 8 8

9. Migration Streams For international migration …
If multiple migration streams play a role in the demography of the country – for example, for countries with significant refugee flows or separate and significant labor and permanent migration streams -- each may need to be modeled separately.
DAPPS and RUP accommodate two separate migration flows:
DAPPS: Primary and Secondary
RUP: “MIG” (international) and “RUM” (internal) 9 9

10. From Estimates to Projections: Base Population Challenges10 10

11. From Estimates to Projections: Base Population Challenges11 11

12. From Estimates to Projections: Base Population Challenges12 12

13. Checking the Base Population
A component projection requires a population properly distributed by sex and age to serve as the base population for the starting date of the projection.
Reliable estimates of the levels of mortality, fertility and migration are required for around the same year.
Usually, the base population is based on data from the latest available census. 13 13

14. Checking the Base Population
But, … census enumerations are not perfect, and the reported data on the population age and sex structure may be affected by underenumeration of persons in certain ages as well as by age misreporting. 14 14

15. Implied coverage of children (undercounts/overcounts) in
East and Southeast Asian censuses
Coverage (in percent)
Census
at age:
Area
year
0-4
5-9
China
1982
-4.2
0.4
-7.0
0.8
1990
-4.8
-2.1
-8.0
2000
-26.2
-12.1
-17.2
-4.4
-20.2
-12.7
Cambodia
1998
-12.5
-3.5
Indonesia
-11.6
-6.1
Japan
-2.3
-1.2
Macau
2001
-5.7
-2.2
Mongolia
-15.9
-7.1
Philippines
-12.6
-3.9
South Korea
-1.9
Sri Lanka
-3.8
Taiwan
0.3
-0.3
Thailand
-5.3
-2.0
Vietnam
1999
-10.2
-0.7

16. Checking the Base Population
During the first years of the projection period, errors in the age and sex composition of the base population may have a large impact on the projected population.
Why?
As noted above, the component method projects the population by following cohorts of persons throughout their lifetimes. 16 16

17. Checking the Base Population
Thus, if the projection starts with errors in the base year, such errors will be carried throughout the projection period and will have an impact on the projected number of births as well. 17 17

18. Checking the Base Population
Suppose, for example, that children age 0 to 4 years were undercounted in the base population.
In the projection, not only would the surviving cohorts of these children be smaller than they should be, but when the female cohorts reach reproductive ages, the number of births they have would also be underestimated. 18 18

19. Checking the Base Population
Consequently, before accepting a population to serve as a base for the projections, an evaluation of:
the completeness of enumeration
the extent of age misreporting
should be undertaken and adjustments made as required. 19 19

20. Checking the Base Population
The two age groups, 0 to 4 years and 5 to 9 years of age, frequently have larger errors than other ages, with the possible exception of the very old.
Errors in the younger ages have a more significant impact on the total projection, and they should be corrected.
In particular, the younger age groups should be checked for consistency with the estimated levels of fertility and mortality. 20 20

21. Checking the Base Population
Errors in the very old ages are not as important,
in the context of cohort component projections.
(Although errors in the very old ages are still important!)
Why?
Because the number of older persons in most populations is relatively small, and these older cohorts will disappear through mortality during the first decades of the projection period. 21 21

22. Checking the Base Population
The BPE procedure entails the following:
The female population 20 to 59 years of age is rejuvenated for the two 5‑year periods prior to the census date to represent the female population in reproductive ages 5 and 10 years earlier.
Male and female births are estimated for the two 5‑year periods prior to the census date.
The two 5-year birth cohorts are re-projected to the census date. These figures represent the adjusted population ages 0 to 4 years and 5 to 9 years at the census date.
If desired by the analyst, results can be adjusted upward to an alternate census total. 22 22

23. Checking the Base Population
BPE does not smooth the population at ages 10 and above. If smoothing is needed, it should be applied before using BPE.
As an analyst, driving the projections, you need to decide whether smoothing is needed, based on your knowledge of the population and data… 23 23

24. Checking the Base Population
BPE inputs
Decimalized reference date pertaining to reported census population
Reported census population by age and sex.
Alternate totals for male and female census population—from a post enumeration survey, for example--are an optional additional input. 24 24

25. Checking the Base Population
Mortality data by age, in the form of the life table population, 5Lx, for at least 2 dates:
An earlier date, preferably at least 7.5 years before the census.
A later date, preferably no earlier than one-half year before the census. 25 25

26. Checking the Base Population
Fertility data in the form of ASFRs, for at least 2 dates:
An earlier date, preferably at least 7.5 years before the census.
A later date, preferably no earlier than one-half year before the census. 26 26

27. Sample Input for BPE: Panel A
Decimalized
reference date
Reported population from
census
Alternate totals for male
and female populations
Sex ratio at birth 27 27

28. Sample Input for BPE: Panel B
The mortality data, in the
form of life table
population, Lx,
for earlier date
The mortality data,
in the form of life table
population, Lx,
for later date 28 28

29. Sample Input for BPE: Panel B
Note: BPE
automatically
Interpolates the three
sets of Lx data needed based on the inputs you provide for two dates. 29 29

30. Sample Input for BPE: Panel C
The fertility data, in the form ASFRs,
for earlier date
The fertility data,
in the form
ASFRs
for later date 30 30

31. Sample Input for BPE: Panel C
Note: BPE automatically interpolates the three sets of ASFRs based on the inputs you provide for two dates. 31 31

32. Sample BPE Results: Panel D
Adjustment 1:
Allows for change to census totals
by sex, in
the process
of developing
alternate
estimates of the 0-4 and 5-9
populations.
Adjustment 2:
Controls population
to alternate
totals by sex,
As input in
Panel A. 32 32

33. Sample BPE Results 33 33

34. Using BPE Output to Build a Base Population
The alternate populations produced by BPE can be used to develop a base population for cohort component projections.
Since BPE inputs often utilize population data pertaining to a census date, and since we need midyear populations to use in cohort-component projections, the BPE output needs to be moved to midyear before serving as a base population for cohort-component projections. 34 34

35. From BPE to MOVEPOP
Output of BPE can serve as input to the MOVEPOP spreadsheet 35 35

36. From BPE to MOVEPOP Net migrants M(x)s Census date and midyear date
Output of BPE
ASFRs 36 36

37. Good choice if it is felt that this is the best quality census.
Base Population Year
Most recent census?
Good choice if it is felt that this is the best quality census.
Uses the most recent data.
Earlier census?
Allows the creation of a population “model” starting at an earlier date.
Options and tradeoffs… 37 37

38. From Estimates to Projections
BPE.xls
ADJAGE.xls
AGEINT.xls
MOVEPOP.xls
AGESMTH.xls
ARRSM5.xls
BEERSP.xls
Pop100h.xls
SP.xls
Base population
Base population adjusted for undercount, consistency with fertility and mortality, smoothed, and moved to midyear
Projection
Fertility
Fertility and projected fertility
Mortality and projected mortality
Mortality
Migration and projected migration
International migration 38 38

39. From Estimates to Projections: Base Population
Spreadsheets for preparing components for projection
Data required
Spreadsheet
Procedure
Age-specific data on population, mortality and fertility.
NewPAS>
BPE.xls
BPE estimates the under 5 population to be consistent with the adult female population, fertility, and mortality.
Population by sex and age
PAS>
AGESMTH.xls
ARRSM5.xls
POP1SM.xls
AGESMTH may be used to smooth 5-year age groups. ARRSM5.xls is similar to the Arriaga method in AGESMTH, but starts at age 5 rather than zero.
POP1SM.xls does a weighted moving average smoothing by single years of age.
Life tables and ASFRs
PAS>SP.xls
SP may be used to construct a stable population where age misreporting is extremely poor.
Population by sex and 5-year ages (plus optional estimate of population under 1)
BEERSP.xls
BEERSP splits the population into single years of age using the Beers osculatory interpolation method. If the population under age 1 is provided, the estimates under age 1 are revised to fit that population. 39 39

40. From Estimates to Projections: Base Population
Spreadsheets for preparing components for projection
Data required
Spreadsheet
Procedure
Age structure and estimated total
PAS>
ADJAGE.xls
ADJAGE adjusts a population to a new total.
Age structure, life tables, ASFRs, and estimated number of migrants
MOVEPOP.xls
MOVEPOP calculates growth rates based on age structure, fertility, mortality, and migration before “moving” the population to midyear.
Population by sex and single years of age
NewPAS>
Pop100h.xls
Pop100h extends a population by single years of age to 100+ based on several possible models.
Population by sex and age at two dates
AGEINT.xls
Interpolate the age distribution to a new date. 40 40

41. Exercises Apply BPE to your census data.
Use the results of BPE (or the enumerated population) in MOVEPOP to move the population to midyear.