The formulae above for the margin of error assume that there is an infinitely large population and thus do not depend on the size of population formula_6, but only on the sample size formula_2. According to sampling theory, this assumption is reasonable when the sampling fraction is small. The margin of error for a particular sampling method is essentially the same regardless of whether the population of interest is the size of a school, city, state, or country, as long as the sampling "fraction" is small.