A new analysis of the gameplay mechanics in Valve's Left 4 Dead series shows how the developers used elements of the Skills, Rules and Knowledge (SRK) framework to measure and control player behavior. The in-depth analysis, written by Full Sail grad student Paul Goodman and course director Adams Greenwood-Ericksen and recently published on Gamasutra, examines how certain Left 4 Dead gameplay elements were carefully designed to make sure players could accurately ascertain the game's rules and develop the knowledge and skills needed to survive. For example, the developer commentary for the first Left 4 Dead reveals a planned Screamer enemy type, that would run off and summon other common zombies if players did not kill it fast enough. While playtesters gained immediate knowledge about both Boomers and Screamers after a single encounter, many found it hard to develop the skills to spot the Screamer effectively or even ascertain the precise rules for how it worked. "It was easier for players to understand how the Boomer worked and develop rules and skills based behaviors to counter it," the pair writes. "Players would learn fairly quickly that it was best engaged at range and that being close to a Boomer when it was killed was not a good strategy. This level of understanding allowed players to handle the Boomer using rule or skills based behaviors." Left 4 Dead 2 expanded on the gameplay framework of the first game, the pair writes, establishing new types of special infected that required alterations of the rules and skills players had learned in the first game. Long narrow hallways, for example, were relatively safe places in the first Left 4 Dead, where players could heal while being covered by their teammates. But the new Charger enemy type could quickly break through this defensive stance, forcing players to re-evaluate their behavior. "Experienced L4D players would need to develop new rules and skill based behaviors regarding their environment and the safety of funneling enemies through doors, as the Charger introduced new risks to using those types of environmental bottlenecks," the pair writes. The full analysis includes discussion of how the game'a AI director helps monitor player's skill level and how breaking up repetitive actions is key to maintaining player interest under the framework.