Bacterial contamination of food. Prior immunization of an individual. Crowding of children in day care facilities. History of infections in an individual. You do an epidemiologic study of cigarette smoking prevalence among women in different countries and the mean age of menopause for women in the same country.
Cigarette smoking is a cause of decreased age at menopause.
There are not enough data points to reach any conclusion. The data suggest that cigarette smoking is associated with increased age of menopause, but a hypothesis testing study is necessary to further evaluate this conclusion. Since there is no prior hypothesis, no conclusion can be made from these data. They could represent just a chance finding of no significance. Which is NOT true about changes in the United States between 1900 and 1990.