The downloadable add-on “Survival Mode” for L4D is apparently out sometime this week, and Valve were kind enough to detail it on their L4D blog, last week. I was off being clever somewhere and so didn’t notice it until this morning. Anyway, it says that “given the extreme pace of Survival Mode, the number of zombies killed in a single round often outnumbers an entire campaign.” It also plots spawn-wave frequency against survival time to create the graph you see above. This, I suspect, is the science of game design. Full thing here.
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More of this sort of thing