Comment by meowface | Hacker News Reader

Comment by meowface | Hacker News Reader

Parent Full thread

>1) False positives lead to under reaction. This is the "boy who cried wolf" issue.

Alert fatigue is a big problem for any alerting system, but here the baseline is 911 reports. If they receive a 911 report for a shooting, they're going to arrive after the report. If they receive a ShotSpotter alert before a 911 report for the same incident, then they're probably going to arrive as soon or sooner than they would have otherwise. If they receive an alert but don't receive a 911 report, then any response time will be faster than no response.

Not sure if these are backed by data, but there are many quotes of law enforcement saying response times are decreased. I could paste a bunch of quotes but I know it doesn't really mean much.

This study used "gunshot victim prehospital time" as a proxy; arguably a more important proxy:

>Use of ShotSpotter detection technology decreases prehospital time for patients sustaining gunshot wounds - https://pubmed.ncbi.nlm.nih.gov/31425474/

Some other studies indeed found it did not seem to decrease response time, though.

Another factor is that it tries to precisely locate the source of the shot. It's hard for humans to infer the location and direction of a supersonic bullet's source if they're going just by hearing and didn't see the incident, while in theory an automated system could determine it more accurately. I don't know how helpful this is empirically, though.

So you may be right. I think most of the value probably comes from cases where they otherwise wouldn't have received a 911 report, or only would have received one much later.

>Alert fatigue is a big problem for any alerting system, but here the baseline is 911 reports. If they receive a 911 report for a shooting, they're going to arrive after the report. If they receive a ShotSpotter alert before a 911 report for the same incident, then they're probably going to arrive as soon or sooner than they would have otherwise. If they receive an alert but don't receive a 911 report, then any response time will be faster than no response.

I'd like to suggest that it might lead to slower response times in the cases of false negatives: someone calls in a report and the ShotSpotter doesn't make an alert. If ShotSpotter tends to be over sensitive police might assume if they don't get an alert from it something definitely didn't occur and take their time

I can see how this could be a risk if it were an assumption they make, but my guess is all PDs would consider human reports far more reliable than ShotSpotter alerts. I'm sure in all or almost all areas, a 911 report of a gunshot is much more likely to be a true positive compared to any automated alert. A lack of a report doesn't imply a lack of an incident, but the presence of a report implies a high likelihood that there is an incident.

A system could be considered generally "sensitive" in some way (e.g. seemingly firing in response to any loud noise) while actually performing poorly in terms of statistical sensitivity (the percentage of actual gunshots that the system detects and alerts on with high confidence).

> If they receive a ShotSpotter alert before a 911 report for the same incident, then they're probably going to arrive as soon or sooner than they would have otherwise.

This seems true, and if it happens a lot that would imply that the system is worthwhile.

> If they receive an alert but don't receive a 911 report, then any response time will be faster than no response.

Assuming it’s real, yes. If it’s not, it’s taken a cop on a wild goose chase when they could have been doing something else.

> Not sure if these are backed by data, but there are many quotes of law enforcement saying response times are decreased. I could paste a bunch of quotes but I know it doesn't really mean much.

Some of these are certainly true and in good faith, but as a general rule I don’t trust quotes from cops about whether or not they need new powers or tools since their answer is always “yes”. Sometimes they’re right, of course, but we need to evaluate more unbiased sources of data to correctly determine what tools are worth it.

It’s a bit like asking me if I need an M1 to do my job better. I might be incidentally right, but I also have extra motivations too.

And that’s before we talk about tools that are more ambiguous. Remember that this data will be used to prosecute people too, if it’s not accurate then the consequences might be worse than just sending out on pointless calls.

> This study used "gunshot victim prehospital time" as a proxy; arguably a more important proxy:

Yeah, that’s compelling.

> Another factor is that it tries to precisely locate the source of the shot. It's hard for humans to infer the location and direction of a supersonic bullet's source if they're going just by hearing and didn't see the incident, while in theory an automated system could determine it more accurately. I don't know how helpful this is empirically, though.

If it works, I agree. The article implies that it is not accurate in determining the location of a gun shot. The extremely high false positive rate also could be from the system accurately detecting a gunshot and inaccurately placing its location.

At a cost of $33 mil a year for the city of Chicago, I personally can’t help but wonder if there’s something else that could be done with that money to reduce the crime rate.

>At a cost of $33 mil a year for the city of Chicago, I personally can’t help but wonder if there’s something else that could be done with that money to reduce the crime rate.

Even if one assumes all the other parts are unreliable, I think the most important question is what percentage of confirmed gunfire was identified by the system which otherwise would never have been discovered, or only discovered more than 30 minutes later.

If there are even a few cases like that, then I think that could lead to someone potentially being saved when they'd otherwise face certain death; potentially due to bleeding out over a long period of time with no one nearby to help. To me, that alone would justify the cost.

This other HN comment is one example:

>For instance, the murder of Seth Rich in my home city (ignore the stupid conspiracy theories) was not followed by a call but was found by shotspotter which is how it was responded to.

Given enough instances like that, it seems pretty plausible that in at least one case, someone who would've died could be rescued.

Also, as a nit: most of the sound from a typical gunshot comes from the bullet “uncorking” from the barrel and suddenly releasing the trapped high-pressure gases all at once, generating a shockwave. This is precisely why suppressors (“silencers”) work, they slow down and cool off these gases before releasing them.

The supersonic crack is a factor, but it’s usually not most of the sound. Typically you will not be able tell a difference between subsonic and supersonic ammunition in the same caliber, it’s all too loud to notice. It’s only once it’s suppressed that you can tell.

Many common handgun calibers are naturally subsonic too, such as the fairly popular .45ACP, as well as heavier 9mm loads. The march 22lr ammo I use is naturally subsonic (faster, trans sonic ammo is less accurate), and it’ll still destroy your hearing without ear protection on.

I know nothing about firearms, but my naive assumption/half-remembered factoid was that the supersonic speed is partly what contributes to what I believed was a common phenomenon of it being difficult to infer where a gun was fired from based on the sound alone. (Not sure if that actually is a common phenomenon or just a trope.)

Maybe any fast, loud noise is difficult to place and the supersonic crack is pretty irrelevant, though? Or maybe sounds tend to just be hard to place in general, similar to how eye witness reports tend to not be very reliable?

Reply on news.ycombinator.com