Thanks, I'll have to look more closely at that paper since I only see discussion of the upper bounds. The reason for my concern is that I noted elsewhere that the prior lower bounds on the merger rates got pretty low: ~0.1 Gpc^-3 yr^-1. For only 16 days of observing the expected number of events drops to 4.38e-3 Gpc^-3.

Then if the horizon distance is somewhat less than than 1 Gpc we need to scale this further. Say it was 0.5 Gpc, we scale by 0.5^3 to get ~5.475e-4 expected events. For 0.2 Gpc horizon we get ~3.5e-5! These values are getting dangerously close to the estimated background rate, at least using this crude calculation at the lower ends of the prior estimates.