Thousands of websites have been created to crowdsource tasks. In this paper, systems that crowdsource human-based computations are organized into four distinct classes using two factors: the users' motivation for completing the task (direct or indirect) and whether task completion is competitive. These classes are described and compared. Considerations and selection criteria for systems designers are presented. This investigation also identified several opportunities for further research. For example, existing systems might benefit from the integration of methods for transforming complex tasks into many simple tasks.
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