1. Period of Projection
The period of projection is 50 years, from 2001 to 2050.
2. Method of Projection
The cohort component method is used for this projection, as with the previous
report. This method takes into consideration international migration while calculating
the ages of the existing population using the future life table. It also uses
the future fertility rate to calculate future births and obtain the number of
survivors for the population that is expected to accrue. Five items, (1) base
population, (2) future survival rate, (3) future fertility rate, (4) future sex
ratio at birth, and (5) future international migration numbers (rates), are required
to project the population using the cohort component method.
3. Base Population
As for the starting point of the projection, called the base population, the male
and female population figures, classified by age group (including non-Japanese
residents) as of October 1, 2000, excerpted from the Population Census of Japan
compiled by the Statistics Bureau, Management and Coordination Agency, were used.
The "age unknown" figure was distributed over all age groups.
4. Assumption of the Survival Rate (Future Life Table)
In order to project the population from one year to the next, survival rates by
age and sex are needed, and, to obtain future survival rates, future life tables
may be constructed.
There are three main ways to construct a future life table: the empirical method,
the mathematical method, and the relational model method. This projection has
adopted the Lee and Carter model, which is based on the relational model, and
modified it to suit the purpose of this projection. The Lee and Carter model describes
age-specific changes of mortality with a single index of mortality by decomposing
a matrix of age-specific death rates into the 'average' mortality age schedule,
the general level of mortality (mortality index), the age-specific changes in
the mortality schedule when the general level of mortality changes, and an error
term. Non-linear curves were fitted to the data after 1970 in order to reflect
the changes in the level of mortality that had gradually been easing off during
the past 30 years. The data for 1995 were excluded due to the influence of the
Great Hanshin Earthquake, the future life tables were constructed separately for
2001 because of the very low reported death counts in February of that year, and
the final fittings were done. The future life tables were constructed from the
assumed age- and sex-specific death rates until 2050, based upon the parameters
obtained through the above procedures.
According to the future life tables, the life expectancy, 77.64 years for males
and 84.62 years for females in the year 2000, is expected to extend to 78.11 years
for males and 85.20 years for females in 2005, 79.76 years for males and 87.52
years for females in 2025, and, in 2050, 80.95 years for males and 89.22 years
for females (see Table 6, Figure
6).
5. Assumptions of Fertility Rates
The age-specific fertility rates are required in order to project the number of
births in future. There are two methods used to estimate future fertility rates:
the period-fertility method and the cohort-fertility method. The latter was adopted
for this projection. The cohort-fertility method observes the birth process per
female birth cohort on an annual basis, and forecasts the level of completed fertility1
and the birth timing for cohorts in which the birth process is incomplete. The
age-specific fertility rate on an annual basis and total fertility rates can be
obtained by converting the estimated cohort fertility data into annual data. Due
to an extensive uncertainty in future fertility, three assumptions (medium, high,
and low variant projections) are compiled and fertility rates are projected for
each of them.
(1) Assumption for the Medium Variant
- The mean age of marriage for cohort has advanced from 24.4 years for the cohort
born in 1950 to 27.8 years for the cohort born in 1985; this tendency will not
change for cohorts born in 2000 and after.
- The proportion never married has advanced from 4.9% for the cohort born in
1950 to 16.8% for the cohort born in 1985; it will not change for cohorts born
in 2000 and after.
- The completed number of births from married persons is affected by later marriage,
later childbearing, and changes in reproductive behavior of couples; it has advanced
from 2.14 for the cohort born between 1948 and 1952 to 1.72 for the cohort born
in 1985; it will not change for cohorts born in 2000 and after.
- The distribution of completed fertility among females appears as follows,
and remains consistent for cohorts born in 2000 and after.
In this case, the total fertility rate will decline from 1.36 in 2000 to 1.31
in 2007. Thereafter, a gradual upward change is predicted, and in 2049 the rate
will be 1.39(see Table 7, Figure
7).
(2) Assumption for the High Variant
- The mean age of marriage for cohort has advanced from 24.4 years for the cohort
born in 1950 to 27.3 years for the cohort born in 1985; this tendency will not
change for cohorts born 2000 and after.
- The proportion never married has advanced from 4.9% for the cohort born in
1950 to 13.3% for the cohort born in 1985; it will not change for cohorts born
in 2000 and after.
- The completed number of births from married persons is affected by later
marriage and later childbearing; it has advanced from 2.14 for the cohort born
between 1948 and 1952 to 1.93 for the cohort born in 1985; it will not change
for cohorts born in 2000 and after.
- The distribution of completed fertility among females appears as follows,
and remains consistent for cohorts born in 2000 and after.
In this case, the total fertility rate will turn upward immediately from 1.36
in 2000, reaching 1.63 in 2049(see Table 7, Figure
7).
(3) Assumption for the Low Variant
- The mean age of marriage for cohort has advanced from 24.4 years for the cohort
born in 1950 to 28.7 years for the cohort born in 1985; this tendency will not
change for cohorts born in 2000 and after.
- The proportion never married has advanced from 4.9% for the cohort born in
1950 to 22.6% for the cohort born in 1985; it will not change for cohorts born
in 2000 and after.
- The completed number of births from married persons is affected by later
marriage, later childbearing, and changes in reproductive behaviors of couples;
it has advanced from 2.14 for the cohort born between 1948 and 1952 to 1.49 for
the cohort born in 1985; it will not change for cohorts born in 2000 and after.
- The distribution of completed fertility among female appears as follows,
and remains consistent for cohorts born in 2000 and after.
In this case, the total fertility rate continues to decline from 1.36 in 2000,
reaching 1.10 in 2049 (see Table 7, Figure
7).
6. Assumption of Sex Ratio at Births
As for the sex ratio at births which divide the future number of newborns into
male and female, the female to male ratio is set to 100:105.5 based on the results
of the last five years, and remains consistent from 2001 onward (see Table 8, Figure
8).
7. Assumption of International Migration
International migration varies according to advances in internationalization and
economic activities of Japan. Additionally, it is affected by the policy concerning
international migration and the economic and social conditions of other countries.
For the past projections of international migration, it was assumed that the
age-specific net (entries minus exits) international migration rate by sex was
constant. However, the international migration trend differs for Japanese and
non-Japanese population. Additionally, migration, especially non-Japanese migration,
does not depend on the population size and the structure of Japanese population.
This projection has different international migration figures for Japanese and
non-Japanese population. That is, two assumptions are made: the age-specific net
international migration rate for Japanese population and the amount of net (entries
minus exits) international migrants for non-Japanese population.
Because international migration for Japanese population is relatively stable,
the assumptions are made as follows: first, obtain the average value of the annual
net international migration rate between 1995 and 2000, and adjust the rate to
remove the blurring which occurs due to random fluctuation as constant for 2001
onward. Because the parent population of migration is Japanese, projection of
the Japanese-only population is required. This population is calculated by multiplying
the projected age- and sex-specific population and the proportion of Japanese
population (obtained from the Population Census of Japan in 2000 and the number
of births in the Vital Statistics).
As for international migration of non-Japanese population, net-migration is more
or less in excess and tends to be increasing, so a regression line is applied
per sex for results from 1970 and after. However, for the years around 1990 when
drastic fluctuations occurred, the years except 1988 to 1995 when the divergence
from the overall tendency is apparent are used and extrapolated using logistic
curves, thereby obtaining the excess net-migrations per sex of non-Japanese population
in the future. The age distribution of net-migrants is fixed as the average value
between 1995 and 2000 (see Figure 9 to
Figure 11).
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