Design effect

In survey research, the design effect is a number that shows how well a sample of people may represent a larger group of people for a specific measure of interest (such as the mean). This is important when the sample comes from a sampling method that is different than just picking people using a simple random sample. The design effect is a positive real number, represented by the symbol Deff. If Deff = 1, then the sample was selected in a way that is just as good as if people were picked randomly. When Deff > 1, then inference from the data collected is not as accurate as it could have been if people were picked randomly. When researchers use complicated methods to pick their sample, they use the design effect to check and adjust their results. It is also used when planning a study in order to figure out how many people should be in the sample.