Used when we want to estimate the proportion of occurrence of some characteristic in the population based on the data collected in the sample. eg find the proportion of people with blood type O in a population. We selected a sample of 300 people and 120 (40%) have blood type O. What is the true proportion of people with blood type O in the population? The result will be something like 95% CI = 34 to 46%. This means that the estimate for the population is between 34 and 46%.