An expansion weight is used so that the weighted data reflects the size of the population of interest (e.g., if there are 300 million people in the population, but 300 people in the survey, a weight of 1,000,000 may be assigned to each respondent).
Expansion weights are also referred to as expansion factors, inflation factors and grossing-up factors. They are typically multiplied with sampling weights to create a single weight. This approach works when the analysis software can accommodate sampling weights but does not work if the analysis program treats weights as frequency weights (because the consequence of the expansion is to inflate the sample size and render all statistical tests invalid).
You can turn sampling weights into expansion weights by dividing the weight variable by the sample size and multiplying by the population size.
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