The OMB further divides core-based statistical areas into metropolitan statistical areas (MSAs) that have "a population of at least 50,000" and micropolitan statistical areas (μSAs) that have "a population of at least 10,000, but less than 50,000."
The OMB classifies core-based statistical areas into metropolitan statistical areas (MSAs) with at least 50,000 inhabitants and micropolitan statistical areas (μSAs) with a minimum population of 10,000 but less than 50,000.