<P> The relative standard error of a sample mean is the standard error divided by the mean and expressed as a percentage . It can only be calculated if the mean is a non-zero value . </P> <P> As an example of the use of the relative standard error, consider two surveys of household income that both result in a sample mean of $50,000 . If one survey has a standard error of $10,000 and the other has a standard error of $5,000, then the relative standard errors are 20% and 10% respectively . The survey with the lower relative standard error can be said to have a more precise measurement, since it has proportionately less sampling variation around the mean . In fact, data organizations often set reliability standards that their data must reach before publication . For example, the U.S. National Center for Health Statistics typically does not report an estimated mean if its relative standard error exceeds 30% . (NCHS also typically requires at least 30 observations--if not more--for an estimate to be reported .) </P>

Standard deviation over the square root of n