To make a population forecast, we must first understand the internal mechanism of population change and establish a population development model, that is, find out the mathematical formula for calculation; Second, we should grasp the population situation in the base period, including various categories of population (gender, age, nationality, urban and rural areas, etc.). ) and the level of various factors (such as fertility, death, migration, etc.) that affect population growth. ); Third, we should estimate the future changes of various elements.
When forecasting population, different population models can be used because of different population conditions or different precision requirements, thus forming different forecasting methods. ① Direct population forecasting method When the population growth rate is basically fixed, the compound interest formula [257-0 1] or exponential growth formula [257-02] can be used to directly calculate the population at a fixed annual growth rate [k] [n] When the expected population growth rate changes, the value of [k] can be adjusted periodically. ② Factor prediction method. According to the correlation ratio, the number of births, deaths, immigrants and immigrants in the next few years are calculated respectively, and then these changes are integrated to predict the changes of the whole population and structure. (3) forecasting method of dividing elements by the same batch of people. Divide the population by sex and age, calculate the number of deaths and corresponding numbers of people of all ages in the future according to the age-specific mortality and survival rate, calculate the number of births in each year according to the number of women of all ages and the age-specific fertility rate, and calculate the number of floating population in each year according to the sex-specific and age-specific mobility rate. The characteristic of this method is to calculate each group of people (1 year-old group or 5-year-old group) separately, and then summarize them, which is not disturbed by the change of age structure. This method is basically used for modern population prediction.
The basic link of the same population factor prediction method is the population age calculation, that is, the population of each age group (or age group) at a certain time point is calculated after one year (or several years). If the death rate by age is relatively stable, then the corresponding survival rate [257- 1 1], that is, the proportion of people who live to+1 year is basically stable, then we can calculate [t]+65438 according to the number of elderly people starting with [t]. For example, we know that the female survival rate of 2 1 is 0.99892, the female population of 1 19485 is 2 1, and the female population of 1988 is [257-65438+]. Other ages count. Just use the number of people born in the previous year [257- 18] and the survival rate of the year of birth [257-2 1] and [257-20] to calculate the number of people in the 0-year-old group at the beginning of each year [257- 17]. The data of survival rate [257- 1 1] can be obtained according to the actual investigation. If there is no actual data, you can also calculate the survival rate in the life table, that is,+1/ instead of [257-1] and 0/0 instead of [257- 19].
The number of deaths in [t] year can be calculated at the same time as the age transfer calculation [257-23]. It is equal to the total number of people who died in [t] years at the beginning of [t] years plus the total number of people born in [t] years [257-24].
[257-03] and [257-04]
The principle of calculating [257- 18] is to multiply the number of women of childbearing age [257-25] at the beginning of [t] by the fertility rate of the corresponding age [257- 10], and add the products to get the expected number of births in [t].
[257-05] Then, the expected number of births is divided into boys and girls according to a certain proportion (such as 0.5 16: 0.484).
In the calculation, the future fertility rate [257- 10] can be treated in many ways: ① it can be assumed that the original age fertility rate [257- 10] will remain unchanged for a long time; ② The total fertility rate [257-06] and fertility distribution [257-27] (also called fertility model) can be calculated according to the original survey [257-26], and the estimated fertility level in the next few years (expressed by the current total fertility rate [257-29]) can be multiplied by the original [257]. ③ The future fertility rate can be calculated according to the birth policy [257-10]; ④ According to certain mathematical formula (such as distribution curve) and given parameter values (such as the age of starting childbearing and the age of peak childbearing), the future fertility rate is estimated [257- 10]. The prediction of migration is based on the above calculation, and then calculated according to the migration rate of gender and age and the corresponding number of people.
The parameters used in the forecast are based on the data obtained from the actual investigation, according to the past trend, referring to the changing process experienced by other countries or regions, or according to the policy requirements, and making assumptions on parameters such as fertility rate and mortality rate. In order to see the differences in the future development of population under different conditions, we can adopt several different sets of assumptions, make different prediction schemes and compare them. Taking family planning as an example, we can choose the most suitable scheme according to different forecast results under different policy requirements. Population forecasting and population planning are inseparable. If there are great differences in fertility and mortality levels between different regions, different ethnic groups or between urban and rural areas, we can and should make predictions separately and then summarize the whole country.
According to the length of the forecast period, population forecast can be divided into short-term forecast (within 5 years), medium-term forecast (5-20 years) and long-term forecast. It can be estimated more accurately, and the calculation result is easy to approach the reality, which has important practical significance. There are many unknown factors in the long-term forecast, which is difficult to grasp, but it can get rid of the influence of short-term fluctuations and show the population development trend, which has important reference value for formulating the population development strategy.