Once you’ve selected the broad research approaches that fit your question, you’ll want to consider a few more defining features for the types of studies that belong in your systematic review.
Here are some other things to determine:
Will there be a minimum sample size for studies included?
Very small studies (say fewer than 10 participants) are often pilot studies and may not have the same rigor as larger ones.
Is there a minimum amount of follow up time with participants?
Some outcomes, especially those related to safety, can’t be detected or measured right away. For example, if you were interested in long-term mortality, any study that followed patients for less than 2 weeks would lack the data you needed to answer your question.
Is there a particular time frame when studies should have been conducted?
Old studies may be out of date because of changes in surgical techniques, standards of care, or other clinical shifts, so it may not make sense to combine information across different time periods.