Population Projection
Griffith Feeney

Objective To test, extend and/or consolidate your understanding of the most basic ideas of demography--age distribution, life tables, age-specific birth rates, and population projection.

Pedagogical Notes This material should be very familiar to some of you but may not be familiar to all of you. Feel free to cooperate in carrying out this exercise. Every participant should try to master all of these very basic concepts, however, and collectively you should produce a minimum of one projection for each two participants. I.e., you may do this exercise in pairs, with each pair of participants producing a projection for a different country.

Many computer programs have been developed for making population projections. The purpose of this exercise is not to produce a projection, however, but to teach basic demographic concepts. This purpose would not be served by using an existing program. Please use Excel to make the projection as indicated below

Step 1 Obtain an initial female age distribution for the projection, either from one of the census publications or from the United Nations Population Division publication that gives age distributions for all countries of the world. See the Syllabus Resources list for this day for the location of a selection of UN age distributions.

Step 2 Put this age distribution in the second column of a a spreadsheet and add age group labels to the first column. If keying in numbers from a census publication, do a sum check to catch keying errors. If importing from a data file, check your spreadsheet against the published figures to insure their accuracy.

Step 3 Obtain a female life table to use for calculating survivorship ratios. Use a life table from a primary source if you can find one. If not, use a Coale-Demeny model life table for females with a suitable expectation of life at birth. See the Syllabus Resources list for this day for the location of the Coale-Demeny tables. You should have an idea of what a suitable female expectation of life at birth for your country is. If necessary, check a statistical yearbook or the UN Population Division publicaiton that gives demographic indicators for every country in the world. Calculate survivorship ratios from this life table and enter them (by copy and paste, of course, not by keying) into the third column of the worksheet.

Step 4 Calculate the survivors of the initial age distribution after five years using the initial distribution and the survivorship ratios. Enter the formula for the projected number of persons 5-9 in the appropriate row of the fourth column of the spreadsheet. Then copy this formula to all lower rows excepting the row for the open-ended age group.

Step 5 Calculate person years lived during the first project period to women aged 15-19, 20-24, ..., 45-49. Put these numbers in the next available column to the right. Use spreadsheet formulas as in the preceding step. Person years lived by women in any age group is 5 times the average of the numbers of women at the beginning and end of the projection period.

Step 6 Obtain a set of age-specific fertility rates appropriate to the population you are working with and enter them immediately to the right of the persons years lived values computed in the preceding step. Apply the rates to the person years lived values to total numers of births to women in each age group, and by addition, total births during the first projection period.

Step 7 Obtain (or decide) a suitable sex ratio at birth. Work out what proportion of total births are female given this sex ratio and apply this proportion to total births to get female births.

Step 8 Compute the survivorship ratio that tells what proportion of births born during a five year period survive to the end of the period and apply this ratio to the female births computed in the last step to obtain the projected number of females aged 0-4 at the end of the first projection period. The first cycle of projection is now complete.

Step 9 Repeat projection cycle to obtain a second projected age distribution ten years into the future. Use Copy and Paste with formulas to minimize additional work.

Step 10 If you know Excel well, re-program the projection calculation so that the projection parameters (survivorship ratios, birth rates, sex ratio at birth) occur in only one location and projected age distributions can be produced in adjacent columns, without intermediate results. Then experiment with longer term projections and observe how the age distribution and population growth rate change over time.

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