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February 4, 2009 - Greg Cherlin

On Tuesday we shipped an update that added a bunch of features / bugfixes / balancing tweaks that came out of the community's feedback. In particular, it made some changes to the underlying TF damage system, and as part of that, it modified the way critical hits are determined. We thought it might be interesting to dig a little into the change, and hopefully give you some insight into our thinking.

First, a quick primer on how the critical hit system works. Each player's chance of successfully rolling for a critical hit depends on two factors:

• A base chance, which is fixed per weapon (2% for all non-melee weapons, 15% for melee)
• An additional bonus, which is based on the amount of damage you've done to enemies in the last 20 seconds. This bonus linearly scales with damage up to a maximum of 10%.

There are two paradigms used for when to roll, and what happens on success:

• Rapid-fire weapons roll for critical hits once per second, instead of every shot. If they roll a crit, the next 2 seconds worth of their fire are marked as critical hits.
• Non-rapid fire weapons roll for critical hits each time you fire, and if they roll a crit, only that shot is marked as a critical hit.
• The sniper rifle and spy knife only score critical hits on headshots and backstabs respectively. They never roll for critical hits.

We had a few things we wanted to change with the old system:

• Reduce the overall number of critical hits occurring in the game.
• Make critical hits more skill / performance based.

Here are the actual changes we made, taken from the release notes:

• Base critical hit chance is now 2% (was 5%)
• Bonus range based on damage done changed from 0%-15% to 0%-10
• Damage range required for bonus changed from 0-1600 to 0-800

Lets dig a little deeper into these. First, the base critical hit chance was reduced from 5% to 2%. This means that if you haven't done any damage to an enemy, your crit chance is now just under half what it was previously. Secondly, the size of the bonus range was reduced by a third, but the amount of damage needed to earn that bonus was halved. To understand the effect of that, it's useful to graph it:

As you can see, the new crit chance is slightly lower across the board, which we wanted. More importantly though, is that the rate at which the crit chance increases based on the amount of recent damage you've done. We like to think of that recent damage total as a rough measure of your performance.

In thinking about the change we wanted to make to critical hits, we decided that there was a point on the graph of particular interest to us, and that was the point at which your critical hit chance was as much a result of your performance as it was the base chance. If you look at at (A) on the old line, you'll see that point isn't reached until you've done 550 recent damage, a feat that occurs about as often as our backstab code works correctly. That point is reached at (B) on the new line, around the point where you've done 175 recent damage. This means that if you've just singlehandedly killed an enemy Demoman/Soldier/Pyro/Heavy, your next 20 seconds worth of crit chances are already more a result of that kill than the base chance. As a result, if you're a highly skilled player, you're going to fire significantly more critical hits than those around you. And remember, if you've just killed 2 or 3 enemies, now's the time to push!