After all, the study was peer-reviewed, right? Doesn't that mean we can trust it?
Here's the thing. Peer review is not perfect. It's not a panacea. It's simply the basic level of due diligence. By submitting work for peer review, a scientist has allowed people outside her own team to critique her work. And the journal might require some changes to the paper based on the critique — anything from edits for clarity to requesting that the scientist perform another experiment in a different way. If a paper hasn't gone through peer review, you should be more skeptical of it. Avoiding peer review means that the researcher decided to show the public her results before allowing those results to be critiqued by independent experts.
But, at the same time, just because something has gone through peer review doesn't mean it's been certified to be accurate. It just means that roughly three other experts have looked at the paper before publication. There's still a lot of room for things to go wrong. Peer review is like the bouncer at the door. The bouncer doesn't guarantee that every person in the bar would be a good person for you to date. Even if a paper gets through, you still have to think about it critically and evaluate it on its own merits. This recent paper on GM corn and rat tumors is an excellent example of that ...
Remember how scientists discovered alien-esque life forms in California and the Internet was all, "Oh, sheeeet!" But then other scientists started critiquing the research and there was a giant debate about whether one scientist could call out another scientist for bad data on a blog, rather than in a peer-reviewed journal, except that the peer reviewed critiques basically said the same thing and the "discovery" turned out to be totally incorrect? I'm making light of arsenic life here just a bit, but this story of de-discovery continues to be interesting and important. Today, on NPR's Science Friday, science journalist extraordinaire Carl Zimmer will explain why, and will talk about what happens when scientists are wrong.
Anything that inspires a good angry rant in real life can be turned into a Downfall video.
Getting a peer reviewed research paper through the aforementioned review process can be a stressful, rant-inducing experience. Remember, in order to be published, the paper is read by three (usually anonymous) reviewers who work in the same field of science. They judge things like whether the experiments described in the paper were done well enough, whether the work is original, and whether the take-away conclusions the scientist is presenting match up with the results of the experiments.