Designers of online communities and social media sites have increasingly been making use of badges as a way to reward users for their achievements. Badges play multiple roles in all these settings. They function as a credentialing system, summarizing the skills and achievements of the individuals who receive them. But they also work powerfully as incentives; experience across many domains shows that people will direct considerable amounts of effort in pursuit of a badge. The question of user participation and contribution in online domains is a broad topic that a number of active lines of research have contributed to.
Badges are a rich language for expressing incentives, but with little existing framework for reasoning about their effects. Our work addresses a set of questions that can help provide insight into badges and their use. Can we find concrete evidence that badges increase site participation or steer users towards taking actions they might not have taken otherwise? If badges do have an effect on users, how can we model user behavior in the presence of badges? And to the extent that designers can indeed steer user behavior with badges, how should they define badges to achieve the outcomes they want?
Theoretical Model of User Behavior. Each badge is defined by specifying how many actions of each type must be performed in order for it to be awarded. A user’s mix of actions over his lifetime can thus be thought of as defining a vector in an n-dimensional space whose ith coordinate records the number of actions of type i that he has taken. Sampling action types from a different distribution incurs a cost based on how far the distribution is from his preferred one.
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