From Social Patterns
In order to make decisions the members or stakeholders of a group need a weight to give their opinions, and project leaders need to know which options have the most support from the participating community.
aka "Polls," "Surveys."
Use this pattern when groups are large enough that only a core subgroup are doing most of the collaboration, to provide a voice to the less fully engaged members of the group.
Provide a form by which a group moderator or participant may suggest a question or topic to be voted on and then a series of possible votes (anything from "yes or no" to a multiple-choice options).
Optionally, provide configuration choices governing such issues as how long the voting will remain open, whether a vote can be changed, whether votes are anonymous or open, whether a person may vote for a single choice or more than one, or if a ranking of choices is preferred.
Voting and surveys provide a means of soliciting feedback about specific questions from a wider participating community.
Note that voting systems can potentially be gamed, especially if no fixed identity is required or authenticated before voting, and voting can provide perverse incentives in much the way that a leaderboard can, and that there are many competing voting algorithms out there, each with their own pros and cons.