A week ago I made a casual statement that in 83% of NFL games, the line doesn’t come into play when determining the winner of the bet. In other words, the winner of the game is either a favorite that covers the spread or an underdog that wins outright 83% of the time. In 17% of the games the favorite wins but doesn’t cover the line. Another way of saying it is that if you picked the winner of every game without paying any attention to the line, you’d win 83% of your bets in the long run.

I know, it sounds outrageous, but I’ll explain how the statistic works and why it doesn’t trouble Vegas.

Anyone who understands Vegas knows that the line is set in order to direct the action 50-50 between the teams (in theory). It does not represent, as many people think, the number of points Team A is superior to Team B. The books make money based on what is called the vigorish (or the vig). Let’s say I bet $11 on team A and you bet $11 on team B. The book holds $22. If team A covers, I collect $21. The other dollar, the vig, is profit for the book. What Vegas is essentially attempting is to predict the division of the action based on giving one team a handicap, also known as the point spread. If New England was playing Cleveland and there was no point spread, the vast majority of the action would be on New England and the books would take a bath if New England won, a pretty good probability.

So back to the 83% figure. This is a pretty well known statistic, but someone on Twitter just went apoplectic with me, saying among other inane things, I think I know more than Vegas. I’d reprint the exchange, but you get the idea. Someone who doesn’t understand something often chooses to berate the person who does understand it. I’ll give you a good example.

I was telling someone that if you shot a gun on a perfectly straight line, and dropped a bullet at the exact same height as the exit opening of the barrel and at the exact same time the bullet exited the barrel, both bullets would hit the ground at the same time. It’s a pretty simply application of the principle in physics that states the force due to gravity is not affected by horizontal velocity. But I might have well as been trying to explain Tourette’s Syndrome to the Salem puritans as an explanation why it wasn’t witchcraft. No way he was buying it. Same with the guy on Twitter. (For all you Sheldon Cooper’s out there, yes, it only works perfectly in a vacuum, but it is close enough in the real world to make the point.) Now admit it – it just doesn’t sound right, does it.

I’ve learned, once the thickness hardens, there is little you can do to penetrate it no matter how hard you try to explain. But here goes once again.

Let’s assume Vegas has done a perfect job and the winner of the game is a favorite that covers 50% of the time, and 50% of the time the underdog wins the bet and there is the exact same amount on both bets. They get all the vig. Of the 50% of the underdogs that collect the bet, two-thirds of them will win the game outright and one-third of them will collect because the favorite won but did not cover. Again, these are statistics you can verify all over the internet. So doing the arithmetic, 50% + (0.67 * 50%) = 83%

So 83% of the time the betting line does not determine the winner of the bet and 17% of the time it does. Pretty simple. If you pick the winner of the game, you’ll only lose 17% of the time. If you think about it, the statistics make perfect sense. So why aren’t people killing Vegas? A few reasons.

First, people like betting the favorite because after all they do win the game 67% (or so) of the time (same arithmetic as above), even if they only collect the bet 50% of the time. People, on average, bet the favorite 65% of the time in Vegas and the underdog 35% of the time. (I’ll avoid a discussion about “trap” lines). Vegas knows this and because they are uncanny about setting the line, they keep the 50-50 division fairly intact. Second, and this is the big one, it’s really hard to pick underdogs that will win outright or cover in 50% of the games. For that matter, it’s really hard to pick the 50% of the favorites that will cover. Just as with horseracing, if you bet the favorite every time, you will be a small loser. If you don’t believe me try it sometime. Millions have and Vegas is still ahead.

Now there are even more broken down statistics available on the internet for percentage of time the favorite wins based on the size of the line. As you might guess, the closer the line, the more likely the underdog will win. In fact, there are public handicappers who base their picks purely on the macro-statistics, and some weeks (or even seasons) they do pretty well, and of course they never do really horrible.

So if you want a system that won’t cost you a ridulous amount of money paid to an “expert” service, look at all the games and identify the 50% of favorites that have the highest probability of winning the game (ignore the line) and bet them. In the other 50% of the games, bet the underdog. And if you are really, really good, you’ll win 83% of your bets.

I’ll await the apology from thickhead, although I’m guessing I’ll wait a long time.