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A review site is a website on which reviews can be posted about people, businesses, products, or services. These sites may use Web 2.0 techniques to gather reviews from site users or may employ professional writers to author reviews on the topic of concern for the site. Early review sites included Epinions.com and Amazon.com.
Review sites are generally supported by advertising. Some business review sites may also allow businesses to pay for enhanced listings, which do not affect the reviews and ratings. Product review sites may be supported by providing affiliate links to the websites that sell the reviewed items.
With the growing popularity of affiliate programs on the Internet, a new sort of review site has emerged - the affiliate product review site. This type of site is usually professionally designed and written to maximize conversions, and is used by e-commerce marketers. It's often based on a blog platform like WordPress, has a privacy and contact page to help with SEO, and has commenting and interactivity turned off. It will also have an e-mail gathering device in the form of an opt-in, or drop-down list to help the aspiring e-commerce business person build an e-mail list to market to.
Because of the specialized marketing thrust of this type of website, the reviews are not objective.
Studies by independent research groups like Forrester Research, comScore, The Kelsey Group, and the Word of Mouth Marketing Association show that rating and review sites influence consumer shopping behavior. In an academic study published in 2008, empirical results demonstrated that the number of online user reviews is a good indicator of the intensity of underlying word-of-mouth effect and increase awareness. In 2007 even large companies such as Best Buy and Walmart began to mention online reviews in television advertisements and on the back of receipts.
Originally reviews were generally anonymous, and most review sites have policies that preclude the release of any identifying information without a court order. Review sites act as public forums, and are legally protected from liability for the content by Section 230 of the Communications Decency Act (CDA).
According to Kurt Opsahl, a staff attorney for the Electronic Frontier Foundation (EFF), anonymity of reviewers is important. "You couldn't have services like ratings sites or Craigslist or message boards or Amazon.com's user feedback or eBay's reviews of sellers without it."
Beginning approximately 2005, however, consumers became more open with their identity and personal information on review sites. Some sites like those from Yelp, Inc. encourage consumers to use their real names, real photos and personal tags.
Most review sites make little or no attempt to restrict postings, or to verify the information in the reviews. Critics point out that positive reviews are sometimes written by the businesses or individuals being reviewed, while negative reviews may be written by competitors, disgruntled employees, or anyone with a grudge against the business being reviewed. So called "reputation management" firms may also submit false positive reviews on behalf of businesses. In 2011, RateMDs.com and Yelp detected dozens of positive reviews of doctors, submitted from the same IP addresses by a firm called Medical Justice.
Furthermore, studies of research methodology have shown that in forums where people are able to post opinions publicly, group polarization often occurs, and the result is very positive comments, very negative comments, and little in between, meaning that those who would have been in the middle are either silent or pulled to one extreme or the other.
Another criticism against sites that rely on income from businesses is that they are reluctant to post negative reviews since that undermines their business model. This leads to a conflict of interest.
Operators of most review sites acknowledge that reviews may not be objective, and that ratings may not be statistically valid. A FAQ  on the Ratingz Inc websites states that, although the ratings are not statistically valid, “They are a listing of opinions and should be judged as such. However, we often receive emails stating that the ratings are uncannily accurate, especially for businesses with over 100 ratings".
Bob Nicholson, a co-founder of Ratingz Inc, goes on to state that "If you get useful information from the ratings, great. That's what we hope happens. If you look at a rating and say, 'Boy, these were obviously all written by the staff in this guy's office', then take it for what it's worth."  PersonRatings.com founder Jeremy Stamper echoes this sentiment, advising site users to "take a person ratings profile with a grain of salt." 
Rating sites have also put in place algorithms to detect patterns of false reviews; it was one of these algorithms that allowed RateMDs' founder John Swapceinski to detect the phoney reviews posted by Medical Justice.
In same cases government authorities have taken legal actions against businesses that post false reviews. In 2009, the State of New York required Lifestyle Lift, a cosmetic surgery company, to pay $300,000 in fines.
In effort to resolve the issue of biased reviews written by the party itself being reviewed, the Reevoo solution verify whether the person writing the review has actually purchased the product.
Aside from sites that enable users to post reviews of products and services, there are also those that work on a "professional" or "expert" basis. Some of these sites commission, and pay for, named individuals or bodies with expertise in a particular field to provide their review material, while others hire in-house editorial staff to perform these reviews. By endeavouring to maintain independence and objectivity and allowing their writers' credentials and site ethos to be scrutinised, such sites avoid many of the above-mentioned criticisms aimed at user-review sites. For example, the UK consumer advocacy organisation, the Consumers' Association, offers Which?, a site that carries no advertising but covers all manner of products and services, with reviews and ratings often based on exhaustive independent testing. Another type of review site does not provide free-form text reviews, but instead evaluates a particular class of products, services, or businesses using a set of pre-defined criteria. One such example is the Canadian shopping site Wishabi, which reviews merchants on 42 criteria and displays the results in a series of icons.