Plants vs. Zombies was chosen because it's "regarded by the gaming community and by critics as a high quality title of broad appeal." The mock reviews used in the study were from five well-known media outlets and participants were told that the aggregate review score they were given was "comprised of 51 professional" outlets. All participants "played the same game, on the same type of computer, in the same environment, for the same amount of time."
An EEDAR representative wrote to Joystiq, "Our ability in the study to influence the participants came only because of the strong trust between consumers and professional media outlets -- the mock reviews in the study were accredited to actual and highly credited media outlets. If for some reason over time professional outlets became biased and graded games that did not match their true quality level, then that trust would deteriorate, and consumers would likely weight other factors higher during the purchasing decision process."
The study also found that high critic reviews "have a strong positive impact on the likelihood of positive word-of-mouth recommendations," continuing that critic reviews "act as a multiplier for the likelihood of a consumer positively recommending the game to a friend." The research also notes that it makes no conclusions on how "marketing, pricing, release timing, brand awareness, and other factors that likely have correlative or causal relationships with video game sales," going so far as to say that those factors could very well influence professional critic reviews (hello, GTA IV).
The EEDAR and SMU study "suggests" that review scores and critic remarks "should be an intricate part of a game's marketing campaign," but it makes no conclusions about the "weight the quantitative (review score) vs. qualitative (critics remarks) had on the participants' results."