TER takes the averages out the looks and performance ratings, and factors in chemistry and location rating to get a total average. A provider has to have a minimum of 20 reviews and the most recent one has to be within the past 3 months.

I guess you're asking about the TER Home Page with the "TER Top 10" lists:
Providers tab | Reviewers tab | Cities drop down
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I think the default sort for Providers is based on "Rating" but some other factors are also involved (the number of recent reviews, etc.). If you check the top left of your own Profile, you see some Rating numbers, sort of like:
"Rating 8.64"
"Looks 8.67 / Performance 8.5"
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"Chemistry 4.35"
"Location 4.5"
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The SINGLE number Rating is not a simple average of (Looks + Performance) / 2 nor (Chemistry + Location) / 2. A few people seem to know the formula. But the key thing is that to be on the Top 10 list you need a High Rating and some other factors. I just don't remember all of the factors and how important each one is (I'm pretty sure that recent reviews is one of them). (All 10s with the most recent review from 2016 won't stay on the Top 10 list.)  
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I think perfectstorm, a sort-of moderator on Newbie might know this stuff. Where's perfectstorm when we need him? Aloha PS! Where are you?