The perspective of human computation argues that combining humans and machines can help tackle increasingly hard problems [1]. As typical evidence, crowdsourcing has emerged as an effective way to perform tasks that are easy for humans but remain difficult for computers. Note that the term crowdsourcing, a form of peer production was coined by Howe [2] to refer to the approach that outsources works to a large group of common people. Crowdsourcing is deemed particularly useful for tasks requiring a large number of viewpoints and problem solvers and, at the same time, it is deemed easy to perceive by the general public [3]. Actually, crowdsourcing platforms, such as Amazon Mechanical Turk (AMT), have enabled the construction of scalable applications for vast tasks ranging from product categorization and photo tagging to audio transcription and translation, and so on.