How do your prevent your main constituency from gaming your (ratings) site?

"Require email verification for accounts. Google even requires an SMS verification; not sure if you want to go that far.
Look for high correlation between matching IP's and identical opinions. Keep a log of all the IP's a user has logged in with. See if those IP's match other users' logs. harrymc explains how this can be circumvented, but maybe geolocating suspicious IP's could show trends indicating abuse. 2 legit cases though: Users are within the same corporate network so they all have the same IP, an ISP that assigns dynamic IP's could give 2 different users the same IP (very rare).
Sometimes you can just smell a rat, in which case, just do a little checking to see if something's amiss.
Sheer volume. The more reviews you get, the harder it will be for any one bogus review to sway the public perception.
Require a certain level of participation before a review is posted. e.g., member for 3 months, 20 posts in the forum (if you have a forum).

As ""A Dwarf"" said, none of this will prevent gaming. The best you can hope is to minimize it. It'll always require some vigilance on your part to constantly adjust tactics: as they begin to understand how you're detecting gaming, they'll try to find ways around it. You find out, they change strategy, etc. ad nauseum."

"You can try and track the answers by their IP address.

If their ISP allocates fixed IP addresses, this would prevent them from answering multiple times from the same connected line.

Even in the case of non-fixed IP, it normally requires one to turn off his router for a certain length of time to be allocated a new IP. As many people today leave their routers on 24/24, with a bit of luck their IP doesn't change that often.

EDIT: This scheme can be defeated if your doctor was sophisticated enough to understand about proxy servers. According to my experience with doctors, there isn't much chance of it."