[strken]

What proportion of the general population's violent crime is caused by substance abuse? If around 40% is attributable to substance abuse[0][1] then you're comparing clean schizophrenics, for whom 0% of crime is attributable to substance abuse, to a population where 40% is.

Note that "attributable" doesn't mean causation, but in reality it usually indicates it to some degree.

[0] https://pubmed.ncbi.nlm.nih.gov/33739484/

[1] https://www.aic.gov.au/publications/tandi/tandi439