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 <a href="/facts/Sampling_design/pg2wgwsg">sampling method</a> that is different than just picking people using a <a href="/facts/Simple_random_sample/vDnCHnNx">simple random sample</a>.
The design effect is a positive <a href="/facts/Real_number/R02gw5Pb">real number</a>, represented by the symbol 
 
 
 
 
 Deff
 
 
 
 {\displaystyle {\text{Deff}}}
 
. If 
 
 
 
 
 Deff
 
 =
 1
 
 
 {\displaystyle {\text{Deff}}=1}
 
, then the sample was selected in a way that is just as good as if people were picked randomly. When 
 
 
 
 
 Deff
 
 >
 1
 
 
 {\displaystyle {\text{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 may also be used when planning a study in order to <a href="/facts/Sample_size_determination/sf8dAFzq">determine the sample size</a>.

Design effect open-in-new

Design effect