Interest in 21st century skills has brought concomitant interest in ways to teach and measure them. Games hold promise in these areas, but much of their potential has yet to be proven, and there are few examples of how to use the rich data from games to make inferences about players' knowledge, skills, and attributes. This article builds an evidence model for the assessment of persistence from Poptropica, a popular commercial game for children. Task persistence is an important skill related to successful school and work outcomes, particularly given new, complex tasks requiring sustained application of effort. Evidence extracted from log files of the game was used to identify players with a particular goal and then create a measure of persistence toward that goal. The results support the ability to create an assessment argument for a game-based measure of persistence.