Suppose you had to choose between two Presidential candidates, one of whom had spent 20 years in Congress plus had considerable other relevant experience and the other of whom had about half a dozen years in the Illinois state legislature and 2 years in Congress. Which one do you think would make a better President? If you chose #1, congratulations, you picked James Buchanan over Abraham Lincoln. Your pick disagrees with that of most historians, who see Lincoln as the greatest President ever and Buchanan as the second worst ever, better only than Warren “Teapot Dome” Harding.
Lots of interesting data and tables and graphs: looks like experience means very, very little when looking at presidential success.