If you're reading this, perhaps you just clicked through from an editors' note on a CNET review of a 2009 TV, looking for a detailed explanation of why we changed the rating. Here it is, in as much excruciating detail as you'd never want to know.
As the page linked from "Detailed editors' rating" below the stars on every CNET review explains, our five-star system is based on some simple math. The number of stars from zero to five correlates to a numeric overall rating from 1 to 10. That rating is determined by three or more subratings, again from 1 to 10, which are assigned weightings and averaged differently for each product category.
In the case of TV reviews those subratings are Design (30 percent), Features (30), and Performance (40). The Sony KDL-NX800, for example, received a 9, 8, 6 respectively, which, when weighted and averaged, works out to 7.5, or 3.5 stars.
So why lower the ratings of older products? The basic reason, which we gave in that editors' note, is "changes in the competitive marketplace." That means new products have been introduced that make the older ones deserve lower ratings.
We could simply leave the old products at the same ratings and rate the newer ones higher, but the obvious downside is that newer products will get ever higher ratings and eventually "break" the scale above. That's not acceptable to us, because we're interested in preserving the sanctity of the 10-point scale; it's much more intuitive than an 11-point, 20-point, or infinity-point scale. The main point of a ratings system, in our view, is to allow easy, intuitive comparison of the general relative merits of multiple products.
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