Online quizzes that determine a person's likely life expectancy use tables of mortality rates associated with specific lifestyle variables and health risks to determine a cumulative likelihood of a person living to a certain age. These tables are based on various sources, including insurance company data and public health studies.
While the exact models behind specific life expectancy quizzes vary, the general principles remain similar across most versions. Some behaviors pose constant risks of death per year that often depend on the amount of the activity a person performs. The most common risk event of this kind is driving, and the odds of a fatal accident are assumed to increase as the person drives more in a given year. Other risks are inherent to certain conditions such as family health history or diet, and are generally considered to increase year over year as someone ages. For example, the increase in odds per year of dying from heart disease for someone with a family history of heart disease is higher than that of someone who has no family history of heart disease.
Once the risks from constant and time-dependent factors are accounted for as regards a person taking the quiz, the software behind the quiz calculates the odds of someone with all of those risk factors surviving for some additional number of years. This calculation is repeated until it reaches the year odds of survival are less than some percentage, often 50 or 25 percent. This number is then reported as the life expectancy of the person taking the quiz.