Does not say how they created their control group. Surveys of people on the street that don't come into the SIS?
What the hell are you even talking about?
"Last year a group of researchers reviewed 47 previous studies."
It's a meta-analysis of a series of studies that all point to the same or similar conclusions. The meta-study lists the various study designs in it:
https://link.springer.com/article/10.1007/s11904-017-0363-ySome are simulations, others are ecological, others time-series, others cross-sectional designs. All of the methods consistently point to the same outcome. SIS reduce
deaths from overdoses and communicable diseases related to IV drug use.
From the study itself:
In the present systematic review, we identified consistent, methodologically sound evidence demonstrating the effectiveness of SCFs in achieving their primary health and public order objectives. Further, the available evidence does not support concerns regarding the potential negative consequences of establishing SCFs, including that these promote drug use or attract crime.