Seen the “98% of studies were ignored!” one doing the rounds on social media. The editorial in the BMJ put it in much better terms:
“One emerging criticism of the Cass review is that it set the methodological bar too high for research to be included in its analysis and discarded too many studies on the basis of quality. In fact, the reality is different: studies in gender medicine fall woefully short in terms of methodological rigour; the methodological bar for gender medicine studies was set too low, generating research findings that are therefore hard to interpret.”
Studies that self select their cohort and don’t include adequate controls are more susceptible to bias than those that do otherwise. Evaluating studies based on their susceptibility to bias is a vital part of the systematic review process.
You can read more about it here https://www.ohri.ca/programs/clinical_epidemiology/oxford.asp
But not actually proof of bias.