National Academies Press: OpenBook

Beyond Six Billion: Forecasting the World's Population (2000)

Chapter: Appendix D: The Effect of Projection Error in Life Expectancy

« Previous: Appendix C: Predicting the Pace of Fertility Decline
Suggested Citation:"Appendix D: The Effect of Projection Error in Life Expectancy." National Research Council. 2000. Beyond Six Billion: Forecasting the World's Population. Washington, DC: The National Academies Press. doi: 10.17226/9828.
×

APPENDIX

D

The Effect of Projection Error in Life Expectancy

Errors in past mortality projections have generally been modest, and we do not expect future errors to be larger. These errors appear not to affect projected population greatly, being more limited in their impact than fertility or migration error. Could mortality errors nevertheless affect some other projected demographic parameters more severely? We consider this question by simulating projections of two hypothetical countries.

METHOD

The first hypothetical country (A) is assumed to resemble Brazil in 1995, with total fertility of 2.5 births and life expectancy for males and females combined of 70 years. The second hypothetical country (B) is assumed to resemble France, with initial total fertility of about 1.5 births and initial combined life expectancy of 78 years.

We project these hypothetical countries as the U.N. does, along trajectories that lead to life expectancies of 84.9 years after 50 years in country A and of 86.6 years after the same period in country B. Total fertility is assumed to follow trajectories, in each country, similar to those applied by the U.N. We assume that these base scenarios represent the true course of demographic trends.

We also produce “error” scenarios for each country by choosing different long-term life expectancy levels. For country A, the alternative is a life expectancy, after 50 years, of 75.7, or 9.2 years below the value in the

Suggested Citation:"Appendix D: The Effect of Projection Error in Life Expectancy." National Research Council. 2000. Beyond Six Billion: Forecasting the World's Population. Washington, DC: The National Academies Press. doi: 10.17226/9828.
×

base projection. For country B, the alternative is a life expectancy, after 50 years, of 81.95, or 4.7 years below the value in the base projection.

Each error scenario involves underestimating future life expectancy, the most common type of error in past projections. These assumed errors are relatively large but are difficult to compare directly with actual errors in recent projections, which have been evaluated for shorter periods only. In 25-year projections, mean error in life expectancy across countries is only 0.52 years. Actual mean absolute error is larger at 4.3 years, but a large part of this can be attributed to absolute error in the base estimate, which averages 1.8 years (see Appendix Table B-5).

RESULTS

Table D-1 shows selected projection outcomes, from each of these scenarios, in projections of 30 and 50 years. The table confirms that the effect on population of error in projected life expectancy is small. Given the larger assumed life expectancy error for Country A than for Country B, the resulting error in projected population is somewhat larger in the former case than in the latter. Raw error in projected death rates is understandably greater, in each case, than error in projected birth rates.

Proportional error in the projected age structure is estimated not from the percentage distributions but from the projected numbers of people. This error affects the elderly much more than the young. In Country A, the number aged 65 and older is underprojected by 15 percent after 30 years, 33 percent after 50 years. The number aged 75 and older is underprojected by 24 percent after 30 years and 41 percent after 50 years. Similar, though smaller proportional errors appear for Country B. However, the dependency ratio is more affected in Country B, being 10 points too low (per hundred workers) in the most extreme case.

The results suggest that, overall, the effects of error in projected life expectancy are generally mild. For particular age groups, however, misspecification can produce larger effects.

Suggested Citation:"Appendix D: The Effect of Projection Error in Life Expectancy." National Research Council. 2000. Beyond Six Billion: Forecasting the World's Population. Washington, DC: The National Academies Press. doi: 10.17226/9828.
×

TABLE D-1 Selected population outcomes in simulated projections with varying gains in life expectancy

 

Crude rates (per 1000)

Percent of population

Country, projection length, and scenario

Population (millions)

Births

Deaths

Under 15.0

65 and older

75 and older

Dependency ratio

Country A after 30 years

Base scenario

237.3

14.9

5.4

21.5

12.6

4.6

51.9

Error scenario

229.9

15.3

7.8

21.9

11.1

3.6

49.2

Error a

−0.031

0.4

2.4

−0.013

−0.147

−0.242

−2.7

Country A after 50 years

Base scenario

273.3

12.6

8.5

19.0

21.0

10.3

66.7

Error scenario

251.3

13.6

11.1

20.1

15.3

6.6

57.9

Error a

−0.080

1.0

2.6

−0.027

−0.330

−0.411

−8.8

Country B after 30 years

Base scenario

62.3

11.1

9.3

16.8

23.6

11.7

67.8

Error scenario

60.8

11.1

11.3

17.2

22.1

10.5

64.8

Error a

−0.024

0.0

2.0

−0.001

−0.086

−0.124

−3.0

Country B after 50 years

Base scenario

62.6

10.3

10.7

16.3

28.4

17.1

81.0

Error scenario

58.9

11.3

13.3

17.0

24.5

13.4

71.0

Error a

−0.059

1.0

2.6

−0.019

−0.188

−0.263

−10.0

aThe difference between the error scenario and the base scenario is given for crude rates and the dependency ratio. For population, proportional error is shown, as it is for population by age group. For age groups, proportional error is based not on the percentage distribution but on the projected numbers of people.

Suggested Citation:"Appendix D: The Effect of Projection Error in Life Expectancy." National Research Council. 2000. Beyond Six Billion: Forecasting the World's Population. Washington, DC: The National Academies Press. doi: 10.17226/9828.
×
Page 315
Suggested Citation:"Appendix D: The Effect of Projection Error in Life Expectancy." National Research Council. 2000. Beyond Six Billion: Forecasting the World's Population. Washington, DC: The National Academies Press. doi: 10.17226/9828.
×
Page 316
Suggested Citation:"Appendix D: The Effect of Projection Error in Life Expectancy." National Research Council. 2000. Beyond Six Billion: Forecasting the World's Population. Washington, DC: The National Academies Press. doi: 10.17226/9828.
×
Page 317
Next: Appendix E: Simluating Migration Projections »
Beyond Six Billion: Forecasting the World's Population Get This Book
×

Is rapid world population growth actually coming to an end? As population growth and its consequences have become front-page issues, projections of slowing growth from such institutions as the United Nations and the World Bank have been called into question.

Beyond Six Billion asks what such projections really say, why they say it, whether they can be trusted, and whether they can be improved. The book includes analysis of how well past U.N. and World Bank projections have panned out, what errors have occurred, and why they have happened.

Focusing on fertility as one key to accurate projections, the committee examines the transition from high, constant fertility to low fertility levels and discusses whether developing countries will eventually attain the very low levels of births now observed in the industrialized world. Other keys to accurate projections, predictions of lengthening life span and of the impact of international migration on specific countries, are also explored in detail.

How good are our methods of population forecasting? How can we cope with the inevitable uncertainty? What population trends can we anticipate? Beyond Six Billion illuminates not only the forces that shape population growth but also the accuracy of the methods we use to quantify these forces and the uncertainty surrounding projections.

The Committee on Population was established by the National Academy of Sciences (NAS) in 1983 to bring the knowledge and methods of the population sciences to bear on major issues of science and public policy. The committee's work includes both basic studies of fertility, health and mortality, and migration; and applied studies aimed at improving programs for the public health and welfare in the United States and in developing countries. The committee also fosters communication among researchers in different disciplines and countries and policy makers in government, international agencies, and private organizations. The work of the committee is made possible by funding from several government agencies and private foundations.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!