If you’re one of those political junkies who really like to get down in the weeds of analysis and observation, chances are you’re already a fan of Nate Silver.

His 538 blog is always an interesting read and today is no exception. Silver statistically analyzes the risk Mitt Romney took in choosing Paul Ryan as his running mate.

Vice-presidential choices are inherently risky to a degree, but the risks are asymmetric, and weighted toward the downside: It’s far easier to name choices who undermined campaigns than those who helped them. The best way to mitigate that downside risk is to select someone who has been tested on a national stage before, ideally by having run for president themselves — or failing that, by having been elected multiple times from a large and diverse state.

Mr. Ryan is a national figure of some repute — before Saturday morning, his national name recognition was about 50 percent — but he has never been elected to anything larger than his Congressional district of about 700,000 people. Members of the House of Representatives have only occasionally been selected as running mates. The last one on a winning ticket was John Nance Garner, the speaker of the House, in 1932. The last time an ordinary member of the House was elected vice president, and the last Republican, was more than 100 years ago: in 1908, when William Howard Taft and James S. Sherman, a New York congressman, were chosen by voters. (Coincidentally, that fall was also the last time that the Chicago Cubs won the World Series.)

Politics 101 suggests that you play toward the center of the electorate. Although this rule has more frequently been violated when it comes to vice-presidential picks, there is evidence that presidential candidates who have more “extreme” ideologies (closer to the left wing or the right wing than the electoral center) underperform relative to the economic fundamentals.

Various statistical measures of Mr. Ryan peg him as being quite conservative. Based on his Congressional voting record, for instance, the statistical system DW-Nominate evaluates him as being roughly as conservative as Representative Michele Bachmann, the controversial congresswoman of Minnesota.

By this measure, in fact, which rates members of the House and Senate throughout different time periods on a common ideology scale, Mr. Ryan is the most conservative Republican member of Congress to be picked for the vice-presidential slot since at least 1900. He is also more conservative than any Democratic nominee was liberal, meaning that he is the furthest from the center. (The statistic does not provide scores for governors and other vice-presidential nominees who never served in Congress.)

Silver’s statistical model gives President Obama a 70% chance of winning at this point. (Intrade has been hovering around 60% for Obama for months. Obama gained 0.3% today.) Faced with stubbornly static polls, Romney needed to do something to shake up the race.

But as Silver points out, while Ryan is a risky choice, he was also an intelligent choice. By no means was Ryan a hail  mary toss or a cry for help. And as Silver points out, it is likely that Romney brought his experience in gauging companies ripe for a turnaround to bear on the decision. With his campaign in trouble, the candidate made a realistic self-assessment and made a rational choice based on all the other factors, including other candidates who were available.

We’ll see over the next few weeks whether Romney chose wisely.