Independence (1 of 5)

Independence of Variables
Two variables are independent if knowledge of the value of one variable provides no information about the value of another variable. For example, if you measured the two variables height and memory ability in a population of adults in the United States, these two variables would in all likelihood be independent. Knowing someone's height would not give you a clue about their memory ability. However, if the variables were height and weight, then there would be a high degree of dependence. When two variables are independent then the Pearson's correlation between them is 0.

Independence of Observations
Two observations are independent if the sampling of one observation does not affect the choice of the second observation. Consider a case in which the observations are not independent: A researcher wishes to estimate how much people engage in shoplifting. The researcher randomly chooses one person to call and interviews him or her. The researcher then asks the person who was just interviewed for the name of a friend so that they can be interviewed next.