There's some subjectivity in the determination of URM, and some objectivity.
The objectivity comes from looking at how many of X there are in the general population, vs. how many X practicing physicians there are.
The US Census doesn't count Arabs (as a poster noted above). But you can figure out the numbers elsewhere. Arabs are about 1% of the US population (
http://www.prejudiceinstitute.org/Factsheets5-ArabAmericans.html) and are about 1.5% of practicing physicians (
http://www.naama.com/naama-arab-american-physicians.php). These numbers could be off by a lot and it still looks like Arabs are not UR (underrepresented).
Comparative numbers compiled from
wikipedia and from the
AMA (precision in these numbers is completely inappropriate as they cross years and sources, but you get the idea):
Whites in the US population: 72.4%
White physicians: 54.5%
Blacks in the US population: 12.6%
Black physicians: 3.5%
Asians in the US population: 4.8%
Asian physicians: 12.2%
Hispanics in the US population: 16.3%
Hispanic physicians: 4.9%
So it's pretty easy to see that Asians, which are M, are not UR, and that Blacks and Hispanics are UR as well as M.
Best of luck to you.