Amazon (NASDAQ:AMZN) is eliminating incentivized reviews from its platform, except for those from within the Amazon Vine program. The change was made to its Community Guidelines and announced today. The changes will apply to all product categories other than books.

This is yet another step in Amazon’s effort to make its review and rating system fairer and more helpful to its customers. Historically, Amazon has prohibited compensation for reviews. It has sued businesses who pay for fake reviews, and has gone after the individuals who write them. Amazon did allow businesses to offer products to customers in exchange for their “honest” review as long as those reviewers disclosed their affiliation with the business in the text of their review.

However, after doing some research, Amazon found that these incentivized reviews tended to be overwhelmingly biased in favor of the product being rated. A study of over 7 million reviews showed that the average rating for products with incentivized reviews was higher than non-incentivized ones. Because of this, incentivized reviews could create top-rated products, raising them from the 54th percentile to the 94th percentile.

The study indicated that reviewers who participated in incentivized reviews had written an average of 232 reviews, while those didn’t participate in them wrote an average of 31 reviews. Reviewers often received the product for free, or at a discounted price, in exchange for their review. Incentivized reviewers also were 12 times less likely to give a 1-star rating than non-incentivized reviews and nearly 4 times less likely to leave a critical review. It is likely that the vendor or seller has sought out reviewers who are less critical. It is also likely that reviewers believed they would lose the opportunity to receive free and discounted items if they chose to post a negative review.

Incentivized reviews from Amazon Vine don’t work the same way. Amazon selects who will be allowed to review products. This would allow Amazon, instead of the seller or vendor, to identify trusted reviewers. Reviewers are only invited to join the program after having written a number of reviews voted as “helpful” by other customers.

The program also has a number of controls in place in order to keep bias out of the review process. Vine reviewers tend to have expertise in a specific product category. Products are submitted directly to Amazon for distribution. Vendors don’t have any contact with Vine reviewers and cannot influence which reviewers will receive their products. Amazon also limits the total number of Vine reviews that are displayed for each product.

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