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Comment by bufferoverflow | Hacker News Reader

You need to divide crime counts by population density. Otherwise you're just plotting population density.

Thanks for checking out the site and for the feedback. It's definitely something we're looking into.

It's kind of an interesting question though because it somewhat varies by crime category. Take car break ins, for instance - this is more of a constant factor than a population density scalar because the number of parking spots is generally fixed on a per-block basis rather than scaling with population density.

I think we might try to give users the option of dividing by population density but allow them to toggle this on/off.

Parking turnover probably scales with population density. One of the reasons parking is more expensive in big cities is to encourage people to move their cars sooner. Smaller cities will have more cars that never move, where larger ones will have fewer.

If it's about movement, then population density might not be the best factor either. You might need some other way to see the number of humans moving through a given area. Stuff like office buildings have higher populations during office hours, but wouldn't be caught in "population" metrics.

Yeah, I think having it as an option is the way to go. Though I’d personally have it default to per-capita, as I do think that correlates closer with actual risk. To illustrate with an extreme example, an area with 10 burglaries per 100 people is clearly a way higher burglary risk place to live than an area with 20 burglaries per 10,000 people.

The number of parking spots may be static, but the number of potential thieves scales with population density.

Does it really? Some classes of crime have been narrowed down to the same 50 individuals. You can't model car break-ins as everybody in the area rolling the dice on getting a taste for breaking glass and grabbing things.

it scales with a factor of less than 1.0 but yes it scales. For instance the number of thieves could be given by the ratio .001 * population.

yes, And this is why it's so frustrating the police don't just setup a few honeypots and catch those 50 people. Sooooooo many citizens would be helped, neighborhoods too, ...

As a toggle would be useful because depending on how you're using the map you might want to see per capita or absolute numbers. (Personally I think it's more useful per capita.)

Car break ins still depend on thievs that scale at some rate in the population. There’s finite time to break into a car.

Divided by population one of highest crime rate areas in the UK is Westminster because this are attracts a lot of tourists (some are targeted by pickpockets) but not much people live there (denominator is small). A better metric will be crime count divided by man-hours spent in the area per day. Such data unlikely exists in open access but mobile operators should be able to estimate it based on base station logs.

I don’t think there’s anything particularly wrong with it. What is it that you think is misleading from this metric?

It's not wrong but what people usually want to know looking at crime stats map is a probability to become a victim if they visit a particular place of will more to live there. Crime count divided by number of residents only relevant for crime targeting residents like home burglaries. For other cases these number can be no-representative.

It should really be crime count by number of people out and about a day, but you will never have that number without some orwellian citizen surveillance program.

Depends on how the map is being used. If I'm looking to avoid traveling through neighborhoods with a high robbery risk (e.g. as a tourist), I'd prefer absolute numbers vs. per capita.

That's probably not the correct analysis, because the number you're trying to estimate is not P(a crime occurs near you), it's P(you will personally be the victim of a crime). Crime rate over density won't necessarily do that either (since the population density of a primarily commercial+touristy area, e.g. Fisherman's Wharf, is << the number of people who are present there on a given day), but neither does raw count.

I mean, I'd also be pretty interested in avoiding P(a crime occurs near you), especially as a tourist.

Yeah there are only 24 hours in a day, so the right metric might be density of crimes in an area per hour. Even if I’m not the victim of a robbery, I don’t really want to be watching it from across the street.

> I'd also be pretty interested in avoiding P(a crime occurs near you), especially as a tourist

What counts as nearby also scales with density. Down the street is nearby in the suburbs. In New York the next avenue block may as well be another town.

As a tourist in SF recently I was interested in seeing the whole city, warts and all, and I did.

I don't really understand why as a tourist you'd want to use this map for a short trip, unless you were going somewhere that's a literal war zone. Most cities just aren't statistically dangerous for a tourist unless you go out looking for trouble. You're much better off applying common sense than digging through digital maps of (reported) crimes.

If I was looking for somewhere to live then it would be more interesting I think.

I dunno, I'd think car break-ins would at least be a very important statistic.

My wife and I will be spending a day in the Fisherman's Wharf area, and the statistics on car break-ins has me concerned, though a co-worker who lives in the Bay told me that if I'm parked in a parking garage, it's less likely I'll be a victim, as it's harder to smash-and-grab-and-go when you have to deal with a parking gate.

if you’re really worried, don’t leave anything in your car and don’t lock it

I don't understand this advice. Are you suggesting that if you make it easier for thieves to break into your car, they are less likely to damage it?

yes and also do not leave anything in your car

Yeah we parked across from the Exploratorium near there for several hours and it was fine, I didn’t think to consult crime heat maps before visiting, just followed standard common sense of not leaving visible ipads etc in the car.

What is the actual car break in statistic there?

This seems like the best use-case, finding a place to live in a city. As a SF-area resident I know the Tenderloin is a no-go zone, but besides that not much. This is a good tool to see which neighborhoods to focus my search on in the apartment-finding process.

>> If I was looking for somewhere to live then it would be more interesting I think.

When I was looking at places to live in South Carolina, Florida and places in the Northwest like Oregon and Idaho, it was interesting to see some of the crime maps and grades these sites gave for crime.

In small coastal towns in Florida, they had small populations and were essentially tourist towns or inhabited by "snow birds" people who live in Northern states who live there during the Winter. Several of the places got D's or F's for crime. I was just shocked, but reading the footnotes, they made it clear that the numbers could be skewed because of the transit population or other factors like Spring Break revelers who come down, get in fights, people get assaulted, or sometimes killed and suddenly a small town's crime rate goes through the roof because of an outlier event that year.

It just made me leery about any statistics you see online with cool heat maps or grades they give to certain areas.

As an aside, the same thing was evident in small rural towns in the mountains in Idaho as well which also got poor grades for crime. It aligns more with your comment, it you go looking for trouble, chances are, you're going to find it.

Yes, data and statistics aren't a substitute for careful thought. You shouldn't throw them out but you definitely can't outsource your thinking to them either.

That's part of why online forums can be nice: a well moderated one can be great at pointing out blind spots in your thinking.

I, as a tourist, would very much prefer being able to walk around looking at random things and relaxing instead of being on the lookout for people trying to steal my things or worse.

Also, when on a trip, getting my phone stolen, for example, is much more of a hassle than having it stolen on the street where I live. Bonus points for thieves actually going for bags and such, so now it's likely you'll need to pay a visit to your embassy or whatever to have your passport replaced, too. In the city where I live, if my ID gets stolen, I'm likely to have at least a different form at home, and I also pretty much know where and when to go to the authorities to try to obtain a replacement.

Speaking of SF, I have no idea if my country even has any kind of representation there, where it would be, how I would get there, and how long it would take to have temporary papers made up, especially without a phone or similar device.

That depends if people care more about crime happening to them or crime happening around them.

this is not a map of population density it’s pretty clear just the SRO strip/TL and then the two mission bart stations

But it's interesting to see that this is not true for all crime categories. E.g. car break-in hotspots are by Fisherman's Wharf and the side of Alamo Square with the Painted Ladies -- i.e. prime tourist locations.

However, I suspect there are some basic data errors upstream of this visualization tool that also limits its usefulness. E.g. 'rape' has a giant hotspot centered at the south side of the intersection of 8th and Mission ... which is site of a city Adult General Assistance office. I am guessing these are being tagged with the location where the victim gave a report, not the location where the crime happened.

And the whole WWII plane meme concern... this is where crimes were caught/reported, not where they occurred.

Yes and no.

Not dividing by population density ~ Probability I will become a crime victim by travelling through that area

Dividing by population densitiy ~ Probability that I would be a crime victim if I live in that area

Not necessarily, people may (or may not? maybe it’s regional) be more likely to victimize strangers.

> Probability I will become a crime victim by travelling through that area

Still not that. P(you’re the victim) = crime count/potential victim count.

Population density may not be the right number for the denominator, but you need something in there.

I could travel through a rural area with an extremely high per capita crime rate but my chances of encountering ANY person (good or bad) is low. I effectively avoid crime by avoiding people.

If you live somewhere, you are forced to be a part of the community, which subjects you to the per capita rate.

And you have to use daytime, not resident, population estimates, or a combination.

Is it actually true that crime in an area increases proportionally with population? Why would it?

Extremely high density but near zero crime would be inside a large office building.

there's such a thing as a "target rich environment" but I always look at these maps as "where people are", college campuses, tourist attractions, crime is high because the number of marks is high, its a coincidence if many people happen to live there too

the issue is that the the specific population density is the density of workers, tourists, and people commuting through the area not the density of people who live there.

As always there is a relevant xkcd: https://xkcd.com/1138/ So many geographic statistics just boil down to population density maps instead.

It's an important truth that walking a mile in a high population density area puts you at a higher crime risk than walking a mile in a medium or low population density area.

Not really, since your chance of being a victim is divided by the number of people in the area.

> Not really, since your chance of being a victim is divided by the number of people in the area.

Depends on behavior. If I stay in my room all day, I don't contribute to your divisor. If Jack Dorsey doesn't walk around alone past midnight, he's not absorbing the same risk quotient either.

> If Jack Dorsey doesn't walk around alone past midnight, he's not absorbing the same risk quotient either.

Jack Dorsey is a weird motherfucker and a real outlier for this example. He used to walk to the Twitter office every day and generally looks and dresses like someone well outside his economic strata. I would not be surprised to find “midnight solo perambulations through the tenderloin” among his hobbies.

It's the opposite - right? There are more gross crimes in the high-density area, but your chance of suffering one is much lower (all else being equal). Trying to apply statistics directly like this is generally inaccurate, but to the degree the numbers say anything it's the opposite of what you say.

Fair point, it's more complicated than I originally thought.

Density increases the number of potential criminals you pass but also increases the number of alternative victims they could target.

If you're a uniquely attractive/visibly wealthy personal walking home from a night out then the risk of passing more criminals dominates the advantage of alterative targets. After all, there's nothing really competing with you.

However, if you look like normal person but happen to have a million dollar watch in your pocket then you're conceivably safer in a city. Because there'd be so many alternative victims who may be more attractive targets than you. Compared to the countryside robber who only gets a single target per night.

There's also a lot of evidence that a given individuals' chance of being the victim of a particular crime derives from their social situation. Most people who regularly do crime are used to doing crime in a way they can repeat, and those crimes most often target locals and people from the same social sphere. There are also plenty of factors that lead to the targeting of tourists or wealthy people, but most crime victims are poor and live in similar conditions to the person committing a crime against them.

Obviously this isn't every crime! People do have unlikely crimes happen to them all the time! Just that they're statistical anomalies and the population level statistics will mislead you about how high your risk is.

The population density in Singapore is much higher than in Detroit. Guess where you are more likely to become the victim of a crime..

Not necessarily, as high population density generally means lots of police and lots of cameras (both security cameras and individuals with smartphones)

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