The first thing that I do (besides discounting most of the 1- and 5-star reviews on Amazon, is to click on the "see all my reviews" link next to the reviewer's location. If the product in question is the only product that s/he has reviewed, I immediately discount it. To me that smacks of either a competitor or a shill, respectively.
I also tend to discount "cheerleaders'" and "naysayers'" (consistently 5 or 1 stars, respectively, for all products reviewed) reviews.
What I do tend to pay more attention to is a review that is atypical for a particular reviewer (e.g., Usually positive or neutral for most products reviewed, but is reviewing this product more negatively). Also, highly detailed reviews earn more credibility from me than, "...the product was stupid" ones.
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