Life Expectancy
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Life expectancy is the average expected number of years of life remaining from a given age, in a given population, according to the current mortality experience (age-specific death rates) of persons in the same population. Life expectancy is calculated from a table called a "Life Table." It is most often expressed as the life expectancy from birth, but is also commonly expressed as life expectancy from age 65.
Life expectancy calculation is based on a "life table," a table of numbers that calculates various
elements for a set of age ranges. The distinctions between types of life expectancy calculation
depend on the characteristics of the life table and the inputs to it.
Source: Anderson, 1999
- A cohort (or generation) life table uses data from a particular birth cohort, for example, the age-specific death rates for all persons born in 1900, after no persons remain alive in the group. (Arias, 2019)
- A period (or current) life table uses current death data, and represents what an hypothetical cohort would experience, given the mortality conditions in the current population. (Arias, 2019)
- The current life table may be used to make statistical inferences and comparisons between the mortality experiences of different populations. (SEPHO, 2005)
- The life table that is calculated in the NJSHAD query system is calculated from a period life table.
- A complete life table calculates life expectancy for every single year of age.
- An abridged life table calculates life expectancy for grouped age intervals, typically 5- or 10-year age groups.
- The abridged method is sometimes used when data are sparsely distributed by single years of age, or when single-year population estimates are not available to compute age-specific death rates.
- Life expectancy that is calculated in the NJSHAD query system is calculated from an abridged life table.
Source: Anderson, 1999
The life expectancy calculations in the NJSHAD
Life Expectancy Query Module use a methodology developed by
Chiang (1968). This methodology was used because it was demonstrated to produce
better estimates of life expectancy for small populations (SEPHO, 2005). The SEPHO
report also demonstrated that models with 5-year age bands to 85+ performed best, and that a population
of 5,000 life years at risk produced an 'acceptable' 95% confidence interval of plus or minus 4 years of
life expectancy.
Where the variance of quantity 'p' is:
Then, assuming that life expectancy is normally distributed, the 95% confidence interval is found by multiplying the standard error by 1.96, where is standard error is the square root of the variance of life expectancy (e).
Sensitive to Infant Mortality
The calculation of life expectancy from birth is sensitive to infant mortality. For that reason, life expectancy from age 65 is often used to compare populations.95% Confidence Interval
Calculation of 95% confidence interval for life expectancy is given in Chiang (1984). The variance of life expectancy (e) may be estimated as:Where the variance of quantity 'p' is:
Then, assuming that life expectancy is normally distributed, the 95% confidence interval is found by multiplying the standard error by 1.96, where is standard error is the square root of the variance of life expectancy (e).
Programming a Life Expectancy Calculator in SAS for NJSHAD
A SAS program written by Zdeb and Dairman (1997) was adapted for use in the NJSHAD Life Expectancy Query Module.
1. Arias E, Xu JQ. United States Life Tables, 2017. National Vital Statistics Reports; vol 68 no 7. Hyattsville, MD: National Center for Health Statistics. 2019.
2. SEPHO: South East Public Health Observatory. Technical Report: Calculating Life Expectancy in Small Areas. November 2005. ISBN 0-954-2971-4-8. Downloaded from http://www.sepho.org.uk/Download/Public/9847/1/Life%20Expectancy%20Nov%2005.pdf on 12/24/2012.
3. Anderson RN. Method for constructing complete annual U.S. life tables. National Center for Health Statistics. Vital Health Stat 2(129). 1999.
4. CL Chiang. The Life Table and Its Construction. In: Introduction to Stochastic Processes in Biostatistics. New York: John Wiley & Sons, 1968:189-214. As cited in SEPHO, 2005.
5. CL Chiang. The Life Table and Its Applications. Malabar, Florida: Robert E. Krieger Publ. Co.; 1984. As cited in Association of Public Health Epidemiologists in Ontario, "10 Life Table Template V1.2." Downloaded from http://www.apheo.ca/index.php?pid=223 on 12/24/2012.
6. Mike Zdeb and Matt Dairman, University at Albany-School of Public Health. Calculating and Illustrating the Probability of Developing Cancer Using SAS and SAS/Graph Software. SAS Users Group International Conference, March 16-19, 1997, San Diego, California; 1997. Downloaded from https://support.sas.com/resources/papers/proceedings/proceedings/sugi22/POSTERS/PAPER260.PDF on 12/24/2012.
2. SEPHO: South East Public Health Observatory. Technical Report: Calculating Life Expectancy in Small Areas. November 2005. ISBN 0-954-2971-4-8. Downloaded from http://www.sepho.org.uk/Download/Public/9847/1/Life%20Expectancy%20Nov%2005.pdf on 12/24/2012.
3. Anderson RN. Method for constructing complete annual U.S. life tables. National Center for Health Statistics. Vital Health Stat 2(129). 1999.
4. CL Chiang. The Life Table and Its Construction. In: Introduction to Stochastic Processes in Biostatistics. New York: John Wiley & Sons, 1968:189-214. As cited in SEPHO, 2005.
5. CL Chiang. The Life Table and Its Applications. Malabar, Florida: Robert E. Krieger Publ. Co.; 1984. As cited in Association of Public Health Epidemiologists in Ontario, "10 Life Table Template V1.2." Downloaded from http://www.apheo.ca/index.php?pid=223 on 12/24/2012.
6. Mike Zdeb and Matt Dairman, University at Albany-School of Public Health. Calculating and Illustrating the Probability of Developing Cancer Using SAS and SAS/Graph Software. SAS Users Group International Conference, March 16-19, 1997, San Diego, California; 1997. Downloaded from https://support.sas.com/resources/papers/proceedings/proceedings/sugi22/POSTERS/PAPER260.PDF on 12/24/2012.
National Center for Health Statistics, Life Tables: https://www.cdc.gov/nchs/products/life_tables.htm
