It’s hard for me to see how they can control for police slowdown. In most cases, in order for there to be a complaint the police have to:

Answer the phone Show up Take the person’a claim seriously, encourage the person to formally complain Actually write up the complaint Actually file it

I’m really skeptical those things wouldn’t be altered by a slowdown, it wouldn’t be much of a slowdown, in my opinion, without altering these things.

Edit: I understand it might be in principle possible to control for a slowdown, if you had many many other slowdowns to use as data points.