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When to shoot twice: XCOM's Rapid Fire probabilities broken down

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You're near the end of an especially hairy XCOM: Enemy Unknown scenario. An alien has one health, and you have an Assault specialist in firing range. Should you use the Rapid Fire ability for two shots at a 15% reduced hit chance, or take one shot? You could make the decision with your gut, or you could do the math, as University of Kent Research Associate Neil Brown has done. In a blog post (opens in new tab) today, Brown answers that question and others with numbers and glowing probability graphs.

The result: If a normal shot has a hit chance between 34% and 96%, the alien only needs one shot to kill, and ammo isn't an issue, you should use Rapid Fire. If one shot isn't enough and you want to do as much damage as possible, you should use Rapid Fire only when your normal hit chance is above 30%.

I won't embarrass myself by attempting to explain Brown's math, but you can see the equations on his website (opens in new tab) . He also (almost definitely correctly) points out that Firaxis must have done the same math when arriving at the 15% Rapid Fire accuracy penalty. That's no surprise, of course, but a reminder that nothing in games is arbitrary. Though I tend to get lost in the surface-level war metaphor and make gut decisions, the best possible decision in any game comes down to the numbers at play—XCOM just puts them on the surface for us to poke at. And by "us," I mean people like Neil Brown, whose mathifying I can enjoy.

Tyler Wilde
Executive Editor

Tyler grew up in Silicon Valley alongside Apple and Microsoft, playing games like Zork and Arkanoid on the early personal computers his parents brought home. He was later captivated by Myst, SimCity, Civilization, Command & Conquer, Bushido Blade (yeah, he had Bleem!), and all the shooters they call "boomer shooters" now. In 2006, Tyler wrote his first professional review of a videogame: Super Dragon Ball Z for the PS2. He thought it was OK. In 2011, he joined PC Gamer, and today he's focused on the site's news coverage. After work, he practices boxing and adds to his 1,200 hours in Rocket League.