But these studies are peer reviewed

You might ask yourself: why do I have to look for bias in these studies? Didn’t the process of peer review weed out sloppy or biased research?

Unfortunately, peer review does not always weed out sloppy or biased research. Published research can still contain significant bias. So, to protect the integrity of your systematic review, you need to closely examine each study, looking for instances where the methods the researchers used might have led to incorrect findings.