When you are betting on football, the spread you get is at least as important as the team that you choose. A lot of research, effort and experience go into setting lines, so they are often very close to the final outcome. The time you put into shopping for the best odds before you make your bet -- even if you can find just a half point difference -- can make the difference between cashing in and crying about what could have been. Shopping lines helps you find the best value that can make you a winner. To do your best shopping, one key fundamental concept you have to be aware of is key numbers.

Key numbers are simply the most common margin of victories in football games. A team can win or lose a game by almost any number of points (my alma mater once lost by 105), but there are some margins that are considerably more likely. The most common key number is 3. In the last decade, about 15 percent of NFL games have been decided by this margin. The reasons for this are obvious - both the last second field goal to win a game and an overtime field goal give us this margin. If a game is tied in the last few minutes you can be fairly sure it will be won by a field goal. In decreasing order of frequency other key numbers include 7, 10, 4, 6. The last two might seem less obvious, but 4 is a touchdown minus a field goal, and 6 is two field goals, a missed conversion, or a touchdown in overtime since they don't kick converts in extra time. 8.7 percent of games in the NFL are decided by 7 points and about 6 percent have a 10 point gap, so just short of one third of games end with a spread of either 3,7 or 10. These numbers have proven more than accurate in recent weeks in the NFL - 6-of-14 games were decided by those gaps in week five, and 5-of-13 in week six.

When it comes to a spread, the key numbers are obviously very important. If a game falls right on the number, then all bettors get their money back, regardless of whish team they bet on. That makes the difference between a 2.5-point spread and a 3-point spread very significant. The former spread will always have an outcome, while in about one-sixth of games money will be refunded on the latter spread. That makes getting in at the right time important. If you like a favorite to win by at least a field goal, you would much prefer a 2.5 spread than a 3 because it gives you that many more chances to win. There is no point in waiting until it moves to 2, because two is not a key number, so the advantage of being at 2 is much less than the disadvantage of being on 3. If you liked the underdog in the same game then you may want to wait to see if the spread will move to the key number so that the favorite has to beat you by more than a field goal.

You only need to look at recent Super Bowls to see the real life impact of key numbers. In the 2000 Super Bowl the Rams started as 7-point favorites, but there was so much action on them that the number moved to 7.5. The game ended up at 23-16. With that move, a bet on St. Louis shifted from a push to a loss, and Tennessee bettors became winners if they waited. The next year, Baltimore was favored by a spread that moved from 2.5 to 3, which may have been an issue if Baltimore hadn't won by 27.

Conservative bettors can look at a key number as a way to hedge their bets. Say you like the favorite and you are fairly confident that they will win by more than a field goal. If the spread was 3.5, you would want to shop around for a 3 point spread. That way, not only do you get to win if your prediction is correct, but you would get your money back in about one game in six. At 3.5 you would lose those games that are won by a field goal, so the key number spread becomes an insurance policy.

For aggressive bettors, or those with deep bankrolls, key numbers, especially three, can create a profitable and low risk situation called middling. Say a spread starts at 2.5, but then must move up to 3 because the action isn't balanced. If the action remains heavy on the favorite the spread would eventually move up again to 3.5. If you can see this kind of situation developing, you would bet an amount on the favorite to win at 2.5 (let's say $110). If that spread eventually moves up to 3.5, you would then bet the same $110 on the underdog. That way, if the game is decided by more or less than three points you will win one bet and lose the other for a total investment of $220 and a payout of $210, or a loss of $10. If the game is won by three, however, you win both bets, so you're $220 bet yields a $200 profit. If you were to find this situation 100 times, statistics would show that you would lose about 85 times, for a loss of $850, but you would win about 15 times, for a profit of $3000 - a very good return for low risk. For this reason, books will stay on a key number longer than they will stay on other numbers to avoid a situation like this that can cause heavy losses.