Daily Fantasy Football: How Line Movement Impacts Defensive Performances

Sunday had finally arrived. I was marching around my house yelling “Can't wait!” like former linebacker Bart Scott. It was Week 16, and it was championship time. 

I was set to face off against my friend Jeff. Our teams were the highest scoring teams in the league, so it was going to be an old-fashioned slobberknocker. 

He and I put forth our best efforts, but by Monday night, he was the victor. 

Anytime you lose a game in fantasy, it helps to review where things went wrong. Amateur players who make mistakes tend to blame it all on luck, injuries, or something else. Those factors certainly matter at times, but not always.

My fatal error that Sunday was playing the wrong defense. I had rostered Houston and Pittsburgh’s defenses for weeks. I held them for that long because of their juicy Week 16 matchups. The Texans were on the road in Tennessee to face Zach Mettenberger, while the Steelers were traveling to Baltimore to face recently-signed Ryan Mallett, who was last seen sulking it up on the sidelines with Houston.

The Titans put up a meager six points on a meaningless touchdown with 1:35 left in the game. They lost 34-6 to the Brandon Weeden-led Texans. 

The Texans defense scored 22 fantasy points.

The Steelers were the biggest Vegas favorites of the weekend, and managed to lose to the Ravens. The Steelers defense scored two fantasy points. You guessed it -- I’m the jabroni who started the Steelers defense. 

Was this failure foreseeable?

Sure. I look at it now and ask, “What the hell was I thinking?” But upon further review, something else jumped out to me: The line movement from Vegas.

Pinnacle Sports, a well-respected sportsbook, opened the Steelers-Ravens game at 47 points on the Monday prior to the game. That's a slightly above average total. Generally, going for defenses with low totals is the right move -- Vegas is predicting a defensive struggle. As the week went on, Mallett was named the starter at quarterback. However, by kickoff, the total actually ticked up to 47.5. So even with the perceived bad news that Mallett was starting, the total moved up. 

This should have raised a red flag.

In the matchup I passed on, Houston and Tennessee opened at 42 points, a relatively low total. But as the week progressed, the total regressed to 39 points. That was tied for the lowest total for the entire season. And there were 256 games played in the regular season, to put that into perspective.

The low total should have persuaded me into taking Houston, sure. But was I missing something else?

What Is Line Movement?

During the NFL season, sportsbooks usually post a spread with a total as early as possible so they can begin taking bets. Generally, if a game is played on Sunday, the line for the following game is posted within the next 24 hours. 

The bookmakers will look at a game like San Diego versus Jacksonville in Week 12, which opened at a 45-point total, but rose two points to 47 by kickoff.

Lines can move for many reasons such as bad weather, injury news, or even coaching decisions. But what moves lines most often is money. Some bettors who excel at betting money -- and plenty of it -- are known as sharp bettors. Sometimes you will hear that “sharp money” is on a certain team, or is on either the "over" or the "under" in a game.

In this game between the Chargers and the Jaguars, the weather was normal and there were no major injuries reported that would have moved the line. Odds are, big money came in on the "over", pushing the total from 45 to 47. The Chargers won the game 31-25, easily eclipsing the 47 point total. 

The Chargers defense, in turn, scored four fantasy points, while the Jaguars scored three. I know, neither team had a good defense this season, but this example shows how a rising total could turn into a shootout of a game.

The Numbers

Now, onto the study.

The combined scores from every FanDuel defense for the regular season averaged out to be 7.44 fantasy points. (That's 512 instances worth of data). And when you break things down by game total, it's pretty evident that low-scoring games -- at least, projected by Vegas -- produce higher defensive fantasy outputs.

But what about line movement? Does the shift in projected game totals affect defensive scoring?

If the total moved up or down 1.50 points or less, on average, there wasn't much of an impact. However, if the line rose two points or higher from open to close, those 50 defenses underachieved, scoring 6.28 FanDuel fantasy points, on average.

On the other end of the spectrum, if you look at what happens when the line drops during the week, the defensive scoring rises. In the 26 instances where the line dropped three points or more, the average defensive score was 8.92, which is more than 1.50 points above league average. That may not seem like a lot, but it is when you consider the range of defensive scores, it's pretty significant.

And in the 46 instances where the line dropped 2.50 points or more, the average outcome was 8.59. Along the same lines, if the line dropped two points or more during the week, our 74 instances show a score of 8.30 points on average. These are all well above average.

In other words, when you're choosing your fantasy defense for the week, don't just pay attention to the game totals. Pay attention to how those game totals came to be, too.