Amazon boasts of the quality and value of the customer-sourced reviews of the products it sells. However, yesterday it announced changes to its community guidelines "to prohibit incentivised reviews unless they are facilitated through the Amazon Vine program". This change should make reviews even more fair and useful for would-be shoppers.
Previously Amazon allowed third party sellers to pay reviewers in discounts or in goods, but not cash, as long as the reviewer disclosed the incentive received within their review copy. It thought of these incentivised reviews were a good way for new products, perhaps from lesser known firms, to gain a market foothold based upon their merit. However, as noted by ReviewMeta, these incentivised reviews tend to be favourable, showing a bias. To fix this issue, from now on, only Amazon Vine program incentivised reviews will be allowed.
You can read about Amazon Vine, and why it should be a trusted source of good quality reviews here. In summary though, this program is based upon reviews from a selection of "the most trusted reviewers on Amazon," as assessed by Amazon using 'reviewer rank', looking at review history 'helpfulness', and other community feedback. Amazon will gift these reviewers new products which Vine program members want to see reviewed. Amazon Vine is an invitation-only programme and negative reviews won't affect the Vine writer's standing, as long as they continue to be helpful to end customers. Importantly vendors have no contact with the 'Vine Voices', and have no influence over which 'Vine Voices' will review their products.
The above policy changes apply immediately to all product categories sold by Amazon, except for books. Amazon will allow "the age-old practice of providing advance review copies of books," to continue.
All charts from ReviewMeta's analysis of 7 million Amazon reviews.