The effect of the 5-rock rule


The 5-rock free guard zone rule has been in effect for 5 seasons. When the rule was introduced there was speculation that it would mainly effect situations where the team with hammer faced a deficit. The ability to have two chances to put up a guard should give the hammer an advantage in generating offense.

But the 5-rock rule has also had benefits for the team throwing first, even in early parts of the game where score effects aren’t a strategic consideration. The team without hammer has been forced to grow up and play with some courage from the very start of the game, with positive results. Let’s enjoy some graphs.

First, let’s look at the hit percentage for each shot of an end before and after the 5-rock rule was adopted. The 4-rock cases include games from the 2014-2018 seasons. The 5-rock cases include games from the 2019 season onward.

(The games included here are from major events from the WCF and Curling Canada. For the WCF, this includes Worlds, the Olympics, and Euros. For Curling Canada, this includes the Brier, Scotties, Canada Cup, Pointsbet Invitational, and Manitoba Championships. This does combine men’s and women’s, but there isn’t a big difference between the two. The women throw slightly more hits.)

I am only including the first 3 ends of the game in this plot so as to minimize score effects. But also because when the rule was adopted, conventional wisdom was that the biggest impacts would be towards the end of the game.

In the left panel we see that under the 4-rock rule, non-hammer threw more hits than hammer (at least after leads’ stones) early in the game. One the right panel we see that under the 5-rock rule, the trend has flipped for the second’s and third’s rocks. Non-hammer is playing more offensive from the beginning of the game, forcing hammer to do more hitting.

Not surprisingly, the fifth stone of the end (non-hammer’s third) has been the shot that has been most impacted. It was a hit 64% of the time under the 4-rock rule, and that has dropped to 42% with the restriction on hitting a guard.

But the downstream and upstream effects are interesting. The 6th and 8th shots used to be hits 61% of the time, but with 5-rock rule that’s gone up to 68%.

That’s a result of there being more stuff to hit – mainly center guards that need clearing. And that’s because early in the game, the main effect of the 5-rock rule has been to make non-hammer more aggressive. Coward’s curling is out and courageous curling is in. At least more often than it used to be. (In case you’re wondering, Krista McCarville still isn’t on board.)

Here’s how the percentage of guards for each of the first 4 shots has changed since the rule was put into effect:

Outside of the 5th shot, the next shot most affected by the rule change has been the first shot. The number of ends that start with a guard skyrocketed from 42% to 57% after the rule change following the 2018 season and has held steady since. That aggressiveness has led to hammer starting with a guard less often as they choose to draw around the guard thrown by their opponent.

Conventional wisdom was that the 5-rock rule would encourage more rocks in play because hammer would be more aggressive. But actually, non-hammer has been more aggressive. Having the last rock is a big advantage, but having the first rock is also an advantage of sorts, and it’s being used to its potential more often.

Under the 5-rock rule, if non-hammer throws a draw on its first shot, there’s still a chance hammer will have a risk-free guard attempt on their second shot that can’t be immediately removed.

We can see this in the data, too. When hammer throws a hit on their first shot (shot 2 of the end) there’s now a much higher chance they will throw a guard on their second shot (shot 4 of the end):

One way to prevent that is to put up a guard on the first shot and immediately force play to the middle.

And more teams are doing that when they’re throwing first. That’s resulted in a more challenging situation for hammer, who by their third shot has had to peel a guard more often than they did in the 4-rock era.