In surveys we usually collect data from a SAMPLE, but we would like to describe what the POPULATION is like. By means of inferential statistics we can estimate what is the value in the population based on the data collected in the sample. eg we want to know 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? eg we want to know the average value of the tensile strength of the metallic alloy X. We made 10 specimens and found an average of 50 kgf/mm2 for this sample. This is the average resistance in the sample, but what would be the average in the population of specimens?