I have set out to make trend maps for every state in the U.S. by taking the margin for each county in 2004, subtracting the national margin, doing the same for 2000, and comparing the numbers.
Say a county was tied in 2004 and 2000. 2004 was Bush +2.47, and 2000 was Gore +0.51. Thus, the county would be 2.47 more Democratic than the national average in 2004, and 0.51 more Republican than the national average in 2000. Thus, a swing of 2.98 to the Democrats.
But to find a swing between the elections, shouldn't you subtract 0.51 from 2.47 instead of adding them together?
