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A reputation system computes and publishes reputation scores for a set of objects (e.g. service providers, services, goods or entities) within a community or domain, based on a collection of opinions that other entities hold about the objects. The opinions are typically passed as ratings to a central place where all perceptions, opinions and ratings accumulated. A reputation center which uses a specific reputation algorithm to dynamically compute the reputation scores based on the received ratings. Reputation is a sign of trustworthiness manifested as testimony by other people.  New expectations and realities about the transparency, availability, and privacy of people and institutions are emerging. Reputation management – the selective exposure of personal information and activities – is an important element to how people function in networks as they establish credentials, build trust with others, and garther information to deal with problems or make decisions. 
Reputation systems are related to recommender systems and collaborative filtering, but with the difference that reputation systems produce scores based on explicit ratings from the community, whereas recommender systems use some external set of entities and events (such as the purchase of books, movies, or music) to generate marketing recommendations to users. The role of reputation systems is to facilitate trust, and often functions by making the reputation more visible.
Types of reputation systems
Online reputation systems
Howard Rheingold states that online reputation systems are 'computer-based technologies that make it possible to manipulate in new and powerful ways an old and essential human trait'. Rheingold inclines that these systems arose as a result of the need for Internet users to gain trust in the individuals they transact with online. The innate trait he makes note of in humans is that a function of society such as gossip 'keeps us up to date on who to trust, who other people trust, who is important, and who decides who is important'. Internet sites such as eBay and Amazon he argues seek to service this consumer trait and are 'built around the contributions of millions of customers, enhanced by reputation systems that police the quality of the content and transactions exchanged through the site'.
The emerging Sharing economy increases the importance of trust in peer-to-peer marketplaces and services. User can build up reputation and trust in individual systems but don’t have the ability to use them in other systems. Rachel Botsman and Roo Rogers argue in their book What’s Mine is Yours (2010), that ‘it is only a matter of time before there is some form of network that aggregates your Reputation capital across multiple form of Collaborative Consumption’. These systems, often referred to as Reputation Banks, try to give users a platform to manage their Reputation capital across multiple systems.
Notable examples of practical applications
- Search: web (see PageRank)
- eCommerce: eBay, Epinions, Bizrate, Trustpilot
- Social news: Reddit, Digg, Imgur
- Programming communities: Advogato, freelance marketplaces, Stack Overflow
- Wikis: Increase contribution quantity and quality (Dencheva, Prause & Prinz 2011)
- Internet Security: TrustedSource
- Question-and-Answer sites: Quora, Yahoo! Answers, Gutefrage.net
- Email: anti-spam techniques, reputation lookup (RapLeaf)
- Personal Reputation: CouchSurfing (for travelers),
- Non Governmental organizations (NGOs): GreatNonProfits.org, GlobalGiving
- Professional reputation of translators and translation outsourcers: BlueBoard at ProZ.com,
- All purpose reputation system: Yelp, Inc.
Attacks on reputation systems
Reputation systems are in general vulnerable to attacks, and many types of attacks are possible. A typical example is the so-called Sybil attack where an attacker subverts the reputation system by creating a large number of pseudonymous entities, and using them to gain a disproportionately large influence. A reputation system's vulnerability to a Sybil attack depends on how cheaply Sybils can be generated, the degree to which the reputation system accepts input from entities that do not have a chain of trust linking them to a trusted entity, and whether the reputation system treats all entities identically.
- Reputation management
- Collaborative filtering
- Web of trust
- Trust metric
- Online participation
- Subjective logic
- Social translucence
- Honor system
- Reputation capital
- Sharing economy
- Commons-based peer production
- Social reputation in fiction
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