Vice presidents who inherited the presidency lack usable first-election data.

Evidence

Presidential appeal was measured by the percentage of popular votes received in their first election to the presidency and by the margin of votes over the second-place candidate. (These two measures intercorrelated +.71, but diverged in years such as 1860, 1912, 1968, and 1980 when there were three or more major candidates.) The four cases where the winning candidate was a former vice president who had taken office on the death of the president (Theodore Roosevelt, Coolidge, Truman, and Lyndon Johnson) were eliminated because these men had not initially gained the presidency in their own right. — John T. Jost and Jim Sidanius, Political Psychology, 1341