> Of course there is inherent bias. There always is. There has to be. How could it be otherwise?
There is an entire field of inquiry dedicated to eliminating "inherent bias" is such studies.
See, just for one small example from 2018:
http://meta-evidence.co.uk/assessing-and-addressing-bias-in-systematic-reviews/
The intricate vastness of the literal army of people who strive to seek knowledge includes a very robust set of schemes and processes that put good guardrails up to keep things like meta-reviews out of the ditches.
Just like statistical analysis, one is well served by consulting with a specialist in the area before attempting one's own meta-analysis, as this is nowhere near as simple a task as it seems.
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