Let’s say you need more evidence to explore your question. Would non-comparative studies be useful for this systematic review?
Case series: Probably not
These studies don’t offer a comparison between treatments, so they’re not useful for your review. You might consider using case series if you don’t have enough information about the outcomes for your intervention or your control.
Case reports: Probably not
These studies aren’t useful because they only talk about an individual patient and offer no treatment comparisons.
If you are really lacking evidence, there’s a chance these individual stories could paint a fuller picture of the outcomes for your intervention or your control.
You’ve picked your study designs so you’re done, right? Not so fast. You still have a few more choices to make.