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On Internet websites which invite users to post comments, a moderation system is the method the webmaster chooses to sort contributions which are irrelevant, obscene, illegal, or insulting with regards to useful or informative contributions.
Various types of Internet sites permit user comments, such as: Internet forums, blogs, and news sites powered by scripts such as phpBB, a Wiki, or PHP-Nuke. Depending on the site's content and intended audience, the webmaster will decide what kinds of user comments are appropriate, then delegate the responsibility of sifting through comments to lesser moderators. Most often, webmasters will attempt to eliminate trolling, spamming, or flaming, although this varies widely from site to site.
Supervisor moderation 
Also known as unilateral moderation, this kind of moderation system is often seen on Internet forums. A group of people are chosen by the webmaster (usually on a long-term basis) to act as delegates, enforcing the community rules on the webmaster's behalf. These moderators are given special privileges to delete or edit others' contributions and/or exclude people based on their e-mail address or IP address, and generally attempt to remove negative contributions throughout the community.
Distributed moderation 
Distributed moderation comes in two types: user moderation and spontaneous moderation.
User moderation 
User moderation allows any user to moderate any other user's contributions. On a large site with a sufficiently large active population, this usually works well, since relatively small numbers of troublemakers are screened out by the votes of the rest of the community. Strictly speaking, wikis such as Wikipedia are the ultimate in user moderation, but in the context of Internet forums, the definitive example of a user moderation system is Slashdot.
For example, each moderator is given a limited number of "mod points," each of which can be used to moderate an individual comment up or down by one point. Comments thus accumulate a score, which is additionally bounded to the range of -1 to 5 points. When viewing the site, a threshold can be chosen from the same scale, and only posts meeting or exceeding that threshold will be displayed. This system is further refined by the concept of karma—the ratings assigned to a user's' previous contributions can bias the initial rating of contributions he or she makes.
On sufficiently specialized websites, user moderation will often lead to groupthink, in which any opinion that is in disagreement with the website's established principles (no matter how sound or well-phrased) will very likely be "modded down" and censored, leading to the perpetuation of the groupthink mentality. This is often confused with trolling.
Spontaneous moderation 
Spontaneous moderation is what occurs when no official moderation scheme exists. Without any ability to moderate comments, users will spontaneously moderate their peers through posting their own comments about others' comments. Because spontaneous moderation exists, no system that allows users to submit their own content can ever go completely without any kind of moderation.
See also 
- Slashdot - A definitive example of user moderation
- Cliff Lampe and Paul Resnick :Slash(dot) and burn: distributed moderation in a large online conversation space Proceedings of the SIGCHI conference on Human factors in computing systems table of contents, Vienna, Austria 2005, 543 - 550.
- Hamed Alhoori, Omar Alvarez, Richard Furuta, Miguel Muñiz, Eduardo Urbina: Supporting the Creation of Scholarly Bibliographies by Communities through Online Reputation Based Social Collaboration. ECDL 2009: 180-191