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The health of the elderly can have a positive or negative impact on average life expectancy. A healthy elderly population will contribute to a higher average, while an elderly population in poor health will do just the opposite.

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Life Expectancy Formulae and Factors

Researchers examine death certificates to calculate life expectancy statistics. From these, they cull age, race, gender, reason for death and other data related to a person's death. When combined with census numbers, these data help calculate an average life expectancy for a national population. Life expectancy data can also be broken down by criteria such as gender, and divided even further into more specific groups as needed.

Life expectancy is a raw statistic, but what factors influence these statistics? A wide array of characteristics -- some unique, others universal -- make a population and its life expectancy what they are. Multidisciplinary population studies help sociologists, economists, psychologists, statisticians and public health professionals learn more about who people are.

The prevalence of­ disease within a population has an effect on the average life expectancy. The World Health Organization (WHO) reports that, in 2005, 38 percent of the adult population of Swaziland, the African nation with the lowest life expectancy in the world, was inflicted with AIDS or HIV. This alone doesn't account for Swaziland's low average life expectancy (38 years for men and 37 years for women), but it has a tremendous impact. Zero percent of the population of Andorra, on the other hand, has AIDS or HIV [source: WHO].

The prevalence of AIDS in Swaziland displays how factors affecting life expectancy are intertwined. Residents of the African nation have less access to health care than Andorrans, in large part be­cause of poverty. Swaziland has a per capita gross domestic product of $5,300, and 69 percent of its residents live below the poverty line.

Poverty also prevents access to safe drinking water and sanitation. Eighty-seven percent of Swaziland's urban population and 42 percent of its rural population has access to safe water. The numbers are similar for access to sanitation. Conversely, 100 percent of both Andorra's rural and urban population has access to safe water and sanitation. One reason it's easier to provide Andorrans with better sanitation is because of the country's size: Fewer people require fewer sewers, and the country saves money on installation and maintenance costs.

Education is another factor that contributes to a longer life expectancy. Only 81 percent of Swaziland's population of more than 1 million people is literate, compared to 100 percent of Andorra's residents. Andorra also has a 100 percent employment rate for its 42,420-person-strong labor force.

So Andorra is smart, small and wealthy. Its people have access to good water, sanitation and health care. But there is at least one other factor that affects the life expectancy of Andorrans: They don't have to worry as much about being killed in an act of god -- their only natural hazards are avalanches.

For more information on populations and age, visit the next page.