Quite frequently marketing studies are based on sample of people or products or stores. The results emerging from such studies are then generalized that is applied to the entire population. For example if a study is done amongst maruti car owners in a city to know their average monthly expenditure on the maintenance of their car it can be done either by covering all maruti car owners residing in that city or by choosing a sample say 10% of the total Maruti car owners. In the latter case the study may give a different average than the actual average if the entire population is covered. This difference between the sample value and the corresponding population value is known as the sampling error.