Analyzing Starcraft 2's Ranking System

Updated on Sunday, July 25, 2010 at 3:03PM by Sirlin

Updated on Sunday, July 25, 2010 at 4:33PM by Sirlin

Let's talk about Starcraft 2's ranking system, specifically the bonus pool system, the focus on ranked matches, and the division system. Before we get into all that, I'll give some background info from three years ago when I talked with Rob Pardo (VP of Game Design at Blizzard) about me possibly taking on the role of ranking-and-tournament systems designer at Blizzard. I ended up having to back out of that race because the Street Fighter HD Remix project suddenly became a reality, and I couldn't pass that up.

The reason I want to tell you about the ranking stuff from back then is to illustrate two points of view--mine and Pardo's--and to explain how it took me three years to understand that his point of view is probably best after all.

TrueSkill vs. Monkeying With Rankings

He asked me to come up with a ranking system for a game, we'll just call it Game X. My first response was that this is easy because it's already a solved problem: Microsoft solved it with what they call TrueSkill. TrueSkill is a refinement of the well-known ELO system used in Chess. One of TrueSkill's main features is that it can apply to games with more than 2 players, while ELO can't. Also, TrueSkill uses a bell curve rather than a single point when referring to a player's skill level. As the system gets more information about the player, it becomes more "certain" that the skill ranking is accurate, and that player's personal bell curve shrinks.

More important than any of those details though, is Microsoft's philosophy about rankings. The premise of their whole system is that players will have the most fun if the ranking system can give matches as close to 50-50 as possible. Yeah it's fun to have to have a few matches that are easy and some that are hard (and you will because of the inevitable variance), but you really do want the ranking system to try to give you close matches. Consider a matchmaking system that simply gave you random opponents, and how from the perspective of a bad player, he just gets stomped repeatedly then probably quits. He is better served by getting evenly matched with other bad players. Over time, he might become a good player rather than quitting.

Microsoft makes another good point here that ONLY winning and losing can be allowed to affect these stats. You can't adjust the matchmaking stat by "experience points" or even by any skill-based stats such as headshots, number of kills, time to finish a lap in racing, etc. All those stats can be gamed, and you will end up trying to get more headshots or something instead of winning. Any formula that equates number of headshots (or any other stat besides wins/losses) with how likely you are to win or lose introduces a layer of imperfect simulation. If we want to know how likely you are to beat someone, we should only consider your wins and losses, and not any in-game stats.

Short version: Microsoft's philosophy is correct (or is it...?), our ranking system should only consider wins and losses and should maximize close matches using TrueSkill, or a close implementation.

But Pardo was not quite on board. Such a ranking system is harsh and while hardcores are ok with it, regular people just get crushed and discouraged. They don't get to see any real progress, he said.

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