Would you prefer they used non-peer-reviewed publications?

Also, it was a metastudy, which can address non-systematic variation in poor studies (it wouldn't necessarily protect you against systematic variation, for example if "all peer reviewed studies were flawed in the same way".

No I would prefer data that was validated by a secondary source. The idea that one can do science by writing a paper and just have it signing off by someone seems flawed and damaging to science and medicine.

Reason I haven't dropped this after being down voted over 5 times. Having had a son who went through the whole cancer deal and then realize that so much of the cancer research is flawed due to bad peer-reviewed papers that caused loss of years of research makes me always question "peer-reviewed." It needs to be replicated and the data needs to be open. Science and especially medical research is seriously damaged by the lack of either one of these.

Your complaint about peer review might be valid but it's irrelevant to this particular study (using the term peer review when describing a study isjust something PR people do to make them seem more valid). Peer review isn't a panacea, but that's not really its role, either. We expect too much of peer review, but it's not directly related to the issues of replication or open data (it certainly doesn't prevent either of those things from happening).

Studies like this actually build on previous studies, and if carried out properly, address many issues about individual publications.

That is why there needs to be more backing then what was published in the letter. The whole backing of data was that it was peer-reviewed.