Life expectancy is a statistical average of the number of years a human is expected to live; this will vary according to region and era. In the Bronze and Iron Age life expectancy was 26; the 2010 world average was 67.2. In Swaziland the average is 49.42 years; in Japan it is 82.6 years. The combination of high infant mortality and deaths in young adulthood from accidents, epidemics, plagues, wars, and childbirth, particularly before modern medicine was widely available, significantly lowers the overall life expectancy. But for those who survive early hazards, a life expectancy of sixty or seventy would not be uncommon. For example, a society with a life expectancy of 40 may have few people dying at age 40: most will die before 30 years of age or after 55. In countries with high infant mortality rates, life expectancy at birth is highly sensitive to the rate of death in the first few years of life. Because of this sensitivity to infant mortality, simple life expectancy at age zero can be subjected to gross misinterpretation, leading one to believe that a population with a low overall life expectancy will necessarily have a small proportion of older people. For example, in a hypothetical stationary population in which half the population dies before the age of five, but everybody else dies at exactly 70 years old, the life expectancy at age zero will be about 36 years, while about 25% of the population will be between the ages of 50 and 70. Another measure, such as life expectancy at age 5 (e5), can be used to exclude the effect of infant mortality to provide a simple measure of overall mortality rates other than in early childhood—in the hypothetical population above, life expectancy at age 5 would be another 65 years. Aggregate population measures, such as the proportion of the population in various age groups, should also be used alongside individual-based measures like formal life expectancy when analyzing population structure and dynamics.