The linked paper does not provide data, so what I do for a similar illustration is grab the lower super output area crime stats from here, and use the 08-17 data to predict homicides in 18-Feb19. I’ve posted the SPSS code I used to do the data munging and graphs here — all the stats could be done in Excel though as well (just involves sorting, cumulative sums, and division). Note this is not quite a replication of the paper, as it includes all cases in the homicide/murder minor crime category, and not just knife crime. There ends up being a total of 147 homicides/murders from 2018 through Feb-2019, so the nature of the task is very similar though, predicting a pretty rare outcome among almost 5,000 lower super output areas (4,831 to be exact).

So the first plot I like to make goes like this. Use whatever metric you want based on *historical* data to rank your areas. So here I used assaults from 08-17. Sort the dataset in descending order based on your prediction. And then calculate the cumulative number of homicides. Then calculate two more columns; the total proportion of homicides your ranking captures given the total proportion of areas.

Easier to show than to say. So for reference your data might look something like below (pretend we have 100 homicides and 1000 areas for a simpler looking table):

```
PriorAssault CurrHom CumHom PropHom PropArea
1000 1 1 1/100 1/1000
987 0 1 1/100 2/1000
962 2 4 4/100 3/1000
920 1 5 5/100 4/1000
. . . . .
. . . . .
. . . . .
0 0 100 100/100 1000/1000
```

You would sort the `PriorCrime`

column, and then calculate `CumHom`

(Cumulative Homicides), `PropHom`

(Proportion of All Homicides) and `PropArea`

(Proportion of All Areas). Then you just plot the `PropArea`

on the X axis, and the `PropHom`

on the Y axis. Here is that plot using the London data.

Paul Ekblom suggests plotting the ROC curve, and I am too lazy now to show it, but it is very similar to the above graph. Basically you can do a weighted ROC curve (so predicting areas with more than 1 homicide get more weight in the graph). (See Mohler and Porter, 2018 for an academic reference to this point.)

Here is the weighted ROC curve that SPSS spits out, I’ve also superimposed the predictions generated via prior homicides. You can see that prior homicides as the predictor is very near the line of equality, suggesting prior homicides are no better than a coin-flip, whereas using all prior assaults does alittle better job, although not great. SPSS gives the area-under-the-curve stat at 0.66 with a standard error of 0.02.

Note that the prediction can be anything, it does not have to be prior crimes. It could be predictions from a regression model (like RTM), see this paper of mine for an example.

So while these do an OK job of showing the overall predictive ability of whatever metric — here they show using assaults are better than random, it isn’t real great evidence that hot spots are the go to strategy. Hot spots policing relies on very targeted enforcement of a small number of areas. The ROC curve shows the entire area. If you need to patrol 1,000 LSOA’s to effectively capture enough crimes to make it worth your while I wouldn’t call that hot spots policing anymore, it is too large.

So another graph you can do is to just plot the cumulative number of crimes you capture versus the total number of areas. Note this is based on the same information as before (using rankings based on assaults), just we are plotting whole numbers instead of proportions. But it drives home the point abit better that you need to go to quite a large number of areas to be able to capture a substantive number of homicides. Here I zoom in the plot to only show the first 800 areas.

So even though the overall curve shows better than random predictive ability, it is unclear to me if a rare homicide event is effectively concentrated enough to justify hot spots policing. Better than random predictions are not necessarily good enough.

A final metric worth making note of is the *Predictive Accuracy Index* (PAI). The PAI is often used in evaluating forecast accuracy, see some of the work of Spencer Chainey or Grant Drawve for some examples. The PAI is simply `% Crime Captured/% Area`

, which we have already calculated in our prior graphs. So you want a value much higher than 1.

While those cited examples again use tables with simple cut-offs, you can make a graph like this to show the PAI metric under different numbers of areas, same as the above plots.

The saw-tooth ends up looking very much like a precision-recall curve, but I haven’t sat down and figured out the equivalence between the two as of yet. It is pretty noisy, but we might have two regimes based on this — target around 30 areas for a PAI of 3-5, or target 150 areas for a PAI of 3. PAI values that low are not something to brag to your grandma about though.

There are other stats like the predictive efficiency index (PAI vs the best possible PAI) and the recapture-rate index that you could do the same types of plots with. But I don’t want to put everyone to sleep.

]]>I have too many PDFs to download them all manually (over 2,000), so I wrote a script in Python to download the PDFs. Unlike prior scraping examples I’ve written about, you need to have signed into your CiteULike account to be able to download the files. Hence I use the *selenium* library to mimic what you do normally in a web-browser.

So let me know what bibliography manager I should switch to. Really one of the main factors will be if I can automate the conversion, including PDFs (even if it just means pointing to where the PDF is stored on my local machine).

This is a good tutorial to know about even if you don’t have anything to do with CiteULike. There are various web services that you need to sign in or mimic the browser like this to download data repeatedly, such as if a PD has a system where you need to input a set of dates to get back crime incidents (and limit the number returned, so you need to do it repeatedly to get a full sample). The selenium library can be used in a similar fashion to this tutorial in that circumstance.

]]>There are two reasons you might want to do this for crime analysis:

- You want to measure the amount of crime around a location, but you rather have a weighted crime count, where crimes closer to the location have a greater weight than those further away.
- You want to measure attributes nearby a location (so things that predict crime), but give a higher weight to those closer to a location.

The second is actually more common in academic literature — see John Hipp’s Egohoods, or Liz Groff’s work on measuring nearby to bars, or Joel Caplan and using kernel density to estimate the effect of crime generators. Jerry Ratcliffe and colleagues work on the buffer intensity calculator is actually the motivation for the original request. So here are some quick code snippets in R to accomplish either. Here is the complete code and original data to replicate.

Here I use over 250,000 reported Part 1 crimes in DC from 08 through 2015, 173 school locations, and 21,506 street units (street segment midpoints and intersections) I constructed for various analyses in DC (all from open data sources) as examples.

First, lets define where our data is located and read in the CSV files (don’t judge me setting the directory, I do not use RStudio!)

```
MyDir <- 'C:\\Users\\axw161530\\Dropbox\\Documents\\BLOG\\buffer_stuff_R\\Code' #Change to location on your machine!
setwd(MyDir)
CrimeData <- read.csv('DC_Crime_08_15.csv')
SchoolLoc <- read.csv('DC_Schools.csv')
```

Now there are several ways to do this, but here is the way I think will be most useful in general for folks in the crime analysis realm. Basically the workflow is this:

- For a given school, calculate the distance between all of the crime points and that school
- Apply whatever function to that distance to get your weight
- Sum up your weights

For the function to the distance there are a bunch of choices (see Jerry’s buffer intensity I linked to previously for some example discussion). I’ve written previously about using the bi-square kernel. So I will illustrate with that.

Here is an example for the first school record in the dataset.

```
#Example for crimes around school, weighted by Bisquare kernel
BiSq_Fun <- function(dist,b){
ifelse(dist < b, ( 1 - (dist/b)^2 )^2, 0)
}
S1 <- t(SchoolLoc[1,2:3])
Dis <- sqrt( (CrimeData$BLOCKXCOORD - S1[1])^2 + (CrimeData$BLOCKYCOORD - S1[2])^2 )
Wgh <- sum( BiSq_Fun(Dis,b=2000) )
```

Then repeat that for all of the locations that you want the buffer intensities, and stuff it in the original `SchoolLoc`

data frame. (Takes less than 30 seconds on my machine.)

```
SchoolLoc$BufWeight <- -1 #Initialize field
#Takes about 30 seconds on my machine
for (i in 1:nrow(SchoolLoc)){
S <- t(SchoolLoc[i,2:3])
Dis <- sqrt( (CrimeData$BLOCKXCOORD - S[1])^2 + (CrimeData$BLOCKYCOORD - S[2])^2 )
SchoolLoc[i,'BufWeight'] <- sum( BiSq_Fun(Dis,b=2000) )
}
```

In this example there are 173 schools and 276,621 crimes. It is too big to create all of the pairwise comparisons at once (which will generate nearly 50 million records), but the looping isn’t too cumbersome and slow to worry about building a KDTree.

One thing to note about this technique is that if the buffers are large (or you have locations nearby one another), one crime can contribute to weighted crimes for multiple places.

To extend this idea to estimating attributes at places just essentially swaps out the crime locations with whatever you want to calculate, ala Liz Groff and her inverse distance weighted bars paper. I will show something alittle different though, in using the weights to create a weighted sum, which is related to John Hipp and Adam Boessen’s idea about Egohoods.

So here for every street unit I’ve created in DC, I want an estimate of the number of students nearby. I not only want to count the number of kids in attendance in schools nearby, but I also want to weight schools that are closer to the street unit by a higher amount.

So here I read in the street unit data. Also I do not have school attendance counts in this dataset, so I just simulate some numbers to illustrate.

```
StreetUnits <- read.csv('DC_StreetUnits.csv')
StreetUnits$SchoolWeight <- -1 #Initialize school weight field
#Adding in random school attendance
SchoolLoc$StudentNum <- round(runif(nrow(SchoolLoc),100,2000))
```

Now it is very similar to the previous example, you just do a weighted sum of the attribute, instead of just counting up the weights. Here for illustration purposes I use a different weighting function, inverse distance weighting with a distance cut-off. (I figured this would need a better data management strategy to be timely, but this loop works quite fast as well, again under a minute on my machine.)

```
#Will use inverse distance weighting with cut-off instead of bi-square
Inv_CutOff <- function(dist,cut){
ifelse(dist < cut, 1/dist, 0)
}
for (i in 1:nrow(StreetUnits)){
SU <- t(StreetUnits[i,2:3])
Dis <- sqrt( (SchoolLoc$XMeters - SU[1])^2 + (SchoolLoc$YMeters - SU[2])^2 )
Weights <- Inv_CutOff(Dis,cut=8000)
StreetUnits[i,'SchoolWeight'] <- sum( Weights*SchoolLoc$StudentNum )
}
```

The same idea could be used for other attributes, like sales volume for restaurants to get a measure of the business of the location (I think more recent work of John Hipp’s uses the number of employees).

Some attributes you may want to do the weighted mean instead of a weighted sum. For example, if you were using estimates of the proportion of residents in poverty, it makes more sense for this measure to be a spatially smoothed mean estimate than a sum. In this case it works exactly the same but you would replace `sum( Weights*SchoolLoc$StudentNum )`

with `sum( Weights*SchoolLoc$StudentNum )/sum(Weights)`

. (You could use the centroid of census block groups in place of the polygon data.)

Using these buffer weights really just swaps out one arbitrary decision for data analysis (the buffer distance) with another (the distance weighting function). Although the weighting function is more complicated, I think it is probably closer to reality for quite a few applications.

Many of these different types of spatial estimates are all related to another (kernel density estimation, geographically weighted regression, kriging). So there are many different ways that you could go about making similar estimates. Not letting the perfect be the enemy of the good, I think what I show here will work quite well for many crime analysis applications.

]]>The trend on the original count scale looks linear, but on the log scale the variance is much nicer. So I’m not sure what the best forecast would be.

I thought the demise had already started earlier in the year, as I actually saw the first year-over-year decreases in June and July. But the views recovered in the following months.

So based on that the slow down in growth I think is a better bet than the linear projection.

For those interested in extending their reach, you should not only consider social media and creating a website/blog, but also writing up your work for a more general newspaper. I wrote an article for The Conversation about some of my work on officer involved shootings in Dallas, and that accumulated nearly 7,000 views within a week of it being published.

Engagement in a greater audience is very bursty. Looking at my statistics for particular articles, it doesn’t make much sense to report average views per day. I tend to get a ton of views on the first few days, and then basically nothing after that. So if I do the top posts by average views per day it is dominated by my more recent posts.

This is partly due to shares on Twitter, which drive short term views, but do not impact longer term views as far as I can tell. That is a popular post on Twitter does not appear to predict consistent views being referred via Google searches. In the past year I get a ratio of about 50~1 referrals from Google vs Twitter, and I did not have *any* posts that had a consistent number of views (most settle in at under 3 views per day after the initial wave). So basically all of my most viewed posts are the same as prior years.

Since I joined Twitter this year, I actually have made fewer blog posts. Not including this post, I’ve made 29 posts in 2018.

```
2011 5
2012 30
2013 40
2014 45
2015 50
2016 40
2017 35
2018 29
```

Some examples of substitution are tweets when a paper is published. I typically do a short write up when I post a working paper — there is not much point of doing another one when it is published online. (To date I have not had a working paper greatly change from the published version in content.) I generally just like sharing nice graphs I am working on. Here is an example of citations over time I just quickly published to Twitter, which was simpler than doing a whole blog post.

Since it is difficult to determine how much engagement I will get for any particular post, it is important to just keep plugging away. Twitter can help a particular post take off (see these examples I wrote about for the Cross Validated Blog), but any one tweet or blog post is more likely to be a dud than anything.

]]>In an effort to advertise a bit more clearly, I wrote a page that describes examples of prior work I have done in collaboration with police departments. That discusses *what* I have previously done, but doesn’t describe *why* a police department would bother to collaborate with me or hire me as a consultant. In fact, it probably makes more sense to contact me for things *no one* has previously done before (including myself).

So here is a more general way to think about (from a police departments or criminal justice agencies perspective) whether it would be beneficial to reach out to me.

So no one is going to be against different evidence based policing practices, but not all strategies make sense for all jurisdictions. For example, while focussed deterrence has been successfully applied in many different cities, if you do not have much of a gang violence problem it probably does not make sense to apply that strategy in your jurisdiction. Implementing any particular strategy should take into consideration the cost as well as the potential benefits of the program.

Should I do X may involve more open ended questions. I’ve previously conducted in person training for crime analysts that goes over various evidence based practices. It also may involve something more specific, such as should I redistrict my police beats? Or I have a theft-from-vehicle problem, what strategies should I implement to reduce them?

I can suggest strategies to implement, or conduct cost-benefit analysis as to whether a specific program is worth it for your jurisdiction.

This is actually the best scenario for me. It is much easier to design a program up front that allows a police department to evaluate its efficacy (such as designing a randomized trial and collecting key measures). I also enjoy tackling some of the nitty-gritty problems of implementing particular strategies more efficiently or developing predictive instruments.

So you want to do hotspots policing? What strategies do you want to do at the hotspots? How many hotspots do you want to target? Those are examples of where it would make sense to collaborate with me. Pretty much all police departments should be doing some type of hot spots policing strategy, but depending on your particular problems (and budget constraints), it will change how you do your hot spots. No budget doesn’t mean you can’t do anything — many strategies can be implemented by shifting your current resources around in particular ways, as opposed to paying for a special unit.

If you are a police department at this stage I can often help identify potential grant funding sources, such as the Smart Policing grants, that can be used to pay for particular elements of the strategy (that have a research component).

Have you done something innovative and want to see if it was effective? Or are you putting a bunch of money into some strategy and are skeptical it works? It is always preferable to design a study up front, but often you can conduct pretty effective post-hoc analysis using quasi-experimental methods to see if some crime reduction strategy works.

If I don’t think you can do a fair evaluation I will say so. For example I don’t think you can do a fair evaluation of chronic offender strategies that use officer intel with matching methods. In that case I would suggest how you can do an experiment going forward to evaluate the efficacy of the program.

Often I collaborate with police departments pro bono — which you may ask what is in it for me then? As an academic I get evaluated mostly by my research productivity, which involves writing peer reviewed papers and getting research grants. So money is not the main factor from my perspective. It is typically easier to write papers about innovative problems or programs. If it involves applying for a grant (on a project I am interested in) I will volunteer my services to help write the grant and design the study.

I could go through my career writing papers without collaborating with police departments. But my work with police departments is more meaningful. It is not zero-sum, I tend to get better ideas when understanding specific agencies problems.

So get in touch if you think I can help your agency!

]]>I am not directly affiliated with any software vendor, so these are my opinions as an outsider, academic, and regular consultant for police departments on quantitative problems.

I do have some academic work on predictive policing applications that folks can peruse at the moment (listed below). The first is on evaluating the accuracy of a people predictions, the second is for addressing the problem of disproportionate minority contact in spatial predictive systems.

- Wheeler, Andrew P., Robert E. Worden, and Jasmine R. Silver. (2018) The predictive accuracy of the Violent Offender Identification Directive (VOID) tool. Conditionally accepted at
*Criminal Justice and Behavior*. Pre-print available here. - Wheeler, Andrew P. (2018)
*Allocating police resources while limiting racial inequality.*Pre-print available here.

I have some more work on predictive policing applications in the pipeline, so just follow the blog or follow me on Twitter for updates about future work.

If police departments are interested in predictive policing applications and would like to ask me some questions, always feel free to get in contact. (My personal email is listed on my CV, my academic email is just Andrew.Wheeler at utdallas.edu.)

Most of my work consulting with police departments is ad-hoc (and much of it is pro bono), so if you think I can be of help always feel free to get in touch. Either for developing predictive applications or evaluating whether they are effective at achieving the outcomes you are interested in.

]]>I’ve previously discussed the idea of using funnel charts to spot outliers, mostly around homicide rates but the idea is the same when examining *any* type of rate. For example in another post I illustrated its use for examining rates of officer involved shootings.

Here is another example applying it to lesser uses of force in New Jersey. Below is the rate of use of force reports per the total number of arrests. (Code to replicate at the end of the post.)

The average use of force per arrests in the state is around 3%. So the error bars show relative to the state average. Here is an interactive chart in which you can use tool tips to see the individual jurisdictions.

Now the original press release noted by Seth Stoughton on twitter noted that several towns have ratio’s of black to white use of force that are very high. Scott Wolfe suspected that was partly a function of smaller towns will have more variable rates. Basically as one is comparing the ratio between two rates with error, the error bars around the rate ratio will also be quite large.

Here is the chart showing the same type of funnel around the rate ratio of black to white use-of-force relative to the average over the whole sample (the black percent use of force is 3.2 percent of arrests, and the white percent use of force is 2.4, and the rate ratio between the two is 1.35). I show in the code how I constructed this, which I should write a blog post about itself, but in short there are decisions I could make to make the intervals wider. So the points that are just slightly above a ratio of 2 at around 10,000 arrests are arguably not outliers, those more to the top-right of the plot though are much better evidence. (I’d note that if one group is very small, you could always make these error bars really large, so to construct them you need to make reasonable assumptions about the size of the two groups you are comparing.)

And here is another interactive chart in which you can view the outliers again. The original press release, Millville, Lakewood, and South Orange are noted as outliers. Using arrests as the denominator instead of population, they each have a rate ratio of around 2. In this chart Millville and Lakewood are outside the bounds, but just barely. South Orange is within the bounds. So those aren’t the places I would have called out according to this chart.

That same twitter thread other folks noted the potential reliability/validity of such data (Pete Moskos and Kyle McLean). These charts cannot say *why* individual agencies are outliers — either high or low. It could be their officers *are* really using force at different rates, it could also be though they are using different definitions to reporting force. There are also potential other individual explanations that explain the use of force distribution as well as the ratio differences in black vs white — no doubt policing in Princeton vs Camden are substantively different. Also even if all individual agencies are doing well, it does not mean there are no potential problem officers (as noted by David Pyrooz, often a few officers contribute to most UoF).

Despite these limitations, I still think there is utility in this type of monitoring though. It is basically a flag to dig deeper when anomalous patterns are spotted. Those unaccounted for factors contribute to more points being pushed outside of my constructed limits (overdispersion), but more clearly indicate when a pattern is so far outside the norm of what is expected the public deserves some explanation of the pattern. Also it highlights when agencies are potentially doing good, and so can be promoted according to their current practices.

This is a terrific start to effectively monitoring police agencies by ProPublica — state criminal justice agencies should be doing this themselves though.

]]>Commonly I want to take public crime data that is in spherical lat/lon coordinates and project it to some local projection. Most of the time so I can do simply euclidean geometry (like buffers within X feet, or distance to the nearest crime generator in meters). Sometimes you need to do the opposite — if I have the projected data and I want to plot the points on a webmap it is easier to work with the lat/lon coordinates. As a note, if you import your map data and then your points are not on the map (or in a way off location), there is some sort of problem with the projection.

I used to do this in ArcMap (toolbox -> Data Management -> Projections), but doing it these programs are faster. Here are examples of going back and forth for some Dallas coordinates. Here is the data and code to replicate the post.

In python there is a library `pyproj`

that does all the work you need. It isn’t part of the default python packages, so you will need to install it using pip or whatever. Basically you just need to define the to/from projections you want. Also it always returns the projected coordinates in meters, so if you want feet you need to do a conversions from meters to feet (or whatever unit you want). For below `p1`

is the definition you want for lat/lon in webmaps (which is not a projection at all). To figure out your local projection though takes a little more work.

To figure out your local projection I typically use this online tool, prj2epsg. You can upload a `prj`

file, which is the locally defined projection file for shapefiles. (It is plain text as well, so you can just open in a text editor and paste into that site as well.) It will then tell you want EPSG code corresponds to your projection.

Below illustrates putting it all together and going back and forth for an example area in Dallas. I tend to write the functions to take one record at a time for use in various workflows, but I am sure someone can write a vectorized version though that will take whole lists that is a better approach.

```
import pyproj
#These functions convert to/from Dallas projection
#In feet to lat/lon
p1 = pyproj.Proj(proj='latlong',datum='WGS84')
p2 = pyproj.Proj(init='epsg:2276') #show how to figure this out, http://spatialreference.org/ref/epsg/ and http://prj2epsg.org/search
met_to_feet = 3.280839895 #http://www.meters-to-feet.com/
#This converts Lat/Lon to projected coordinates
def DallConvProj(Lat,Lon):
#always returns in meters
if abs(Lat) > 180 or abs(Lon) > 180:
return (None,None)
else:
x,y = pyproj.transform(p1, p2, Lon, Lat)
return (x*met_to_feet, y*met_to_feet)
#This does the opposite, coverts projected to lat/lon
def DallConvSph(X,Y):
if abs(X) < 2000000 or abs(Y) < 6000000:
return (None,None)
else:
Lon,Lat = pyproj.transform(p2, p1, X/met_to_feet, Y/met_to_feet)
return (Lon, Lat)
#check coordinates
x1 = -96.828295; y1 = 32.832521
print DallConvProj(Lat=y1,Lon=x1)
x2 = 2481939.934525765; y2 = 6989916.200679892
print DallConvSph(X=x2, Y=y2)
```

In R I use the library `proj4`

to do the projections for point data. R can read in the projection data from a file as well using the `rgdal`

library.

```
library(proj4)
library(rgdal)
#read in projection from shapefile
MyDir <- "C:\\Users\\axw161530\\Dropbox\\Documents\\BLOG\\Projections_R_Python"
setwd(MyDir)
DalBound <- readOGR(dsn="DallasBoundary_Proj.shp",layer="DallasBoundary_Proj")
DalProj <- proj4string(DalBound)
ProjData <- data.frame(x=c(2481939.934525765),
y=c(6989916.200679892),
lat=c(32.832521),
lon=c(-96.828295))
LatLon <- proj4::project(as.matrix(ProjData[,c('x','y')]), proj=DalProj, inverse=TRUE)
#check to see if true
cbind(ProjData[,c('lon','lat')],as.data.frame(LatLon))
XYFeet <- proj4::project(as.matrix(ProjData[,c('lon','lat')]), proj=DalProj)
cbind(ProjData[,c('x','y')],XYFeet)
plot(DalBound)
points(ProjData$x,ProjData$y,col='red',pch=19,cex=2)
```

The last plot function shows that the XY point is within the Dallas basemap for the projected boundary. But if you want to project the boundary file as well, you can use the `spTransform`

function. Here I have a simple example of tacking the projected boundary file and transforming to lat/lon, so can be superimposed on a leaflet map.

Additionally I show a trick I sometimes use for maps by transforming the boundary polygon to a polyline, as it provides easier styling options sometimes.

```
#transform boundary to lat/lon
DalLatLon <- spTransform(DalBound,CRS("+init=epsg:4326") )
plot(DalLatLon)
points(ProjData$lon,ProjData$lat,col='red',pch=19,cex=2)
#Leaflet useful for boundaries to be lines instead of areas
DallLine <- as(DalLatLon, 'SpatialLines')
library(leaflet)
BaseMapDallas <- leaflet() %>%
addProviderTiles(providers$OpenStreetMap, group = "Open Street Map") %>%
addProviderTiles(providers$CartoDB.Positron, group = "CartoDB Lite") %>%
addPolylines(data=DallLine, color='black', weight=4, group="Dallas Boundary Lines") %>%
addPolygons(data=DalLatLon,color = "#1717A1", weight = 1, smoothFactor = 0.5,
opacity = 1.0, fillOpacity = 0.5, group="Dallas Boundary Area") %>%
addLayersControl(baseGroups = c("Open Street Map","CartoDB Lite"),
overlayGroups = c("Dallas Boundary Area","Dallas Boundary Lines"),
options = layersControlOptions(collapsed = FALSE)) %>%
hideGroup("Dallas Boundary Lines")
BaseMapDallas
```

I have too much stuff in the blog queue at the moment, but hopefully I get some time to write up my notes on using leaflet maps in R soon.

]]>Here is a graph illustrating the problem. X axis is the proportion of minorities stopped by the police in 500 by 500 meter grid cells (NYPD data). Y axis is the number of violent crimes over along time period (12 years). So a typical hot spots strategy would choose the top N areas to target (here I do top 20). These are all very high proportion minority areas. So the inevitable extra police contact in those hot spots (in the form of either stops or arrests) will *increase* DMC.

I’d note that the majority of critiques of predictive policing focus on whether reported crime data is biased or not. I think that is a bit of a red herring though, you could use totally objective crime data (say swap out acoustic gun shot sensors with reported crime) and you still have the same problem.

The proportion of stops by the NYPD of minorities has consistently hovered around 90%, so doing a bunch of extra stuff in those hot spots will increase DMC, as those 20 hot spots tend to have 95%+ stops of minorities (with the exception of one location). Also note this 90% has not changed even with the dramatic decrease in stops overall by the NYPD.

So to illustrate my suggested solution here is a simple example. Consider you have a hot spot with predicted 30 crimes vs a hot spot with predicted 28 crimes. Also imagine that the 30 crime hot spot results in around 90% stops of minorities, whereas the 28 crime hot spot only results in around 50% stops of minorities. If you agree reducing DMC is a reasonable goal for the police in-and-of-itself, you may say choosing the 28 crime area is a good idea, even though it is a less efficient choice than the 30 crime hot spot.

I show in the paper how to codify this trade-off into a linear program that says choose X hot spots, but has a constraint based on the expected number of minorities likely to be stopped. Here is an example graph that shows it doesn’t always choose the highest crime areas to meet that racial equity constraint.

This results in a trade-off of efficiency though. Going back to the original hypothetical, trading off a 28 crime vs 30 crime area is not a big deal. But if the trade off was 3 crimes vs 30 that is a bigger deal. In this example I show that getting to 80% stops of minorities (NYC is around 70% minorities) results in hot spots with around 55% of the crime compared to the no constraint hot spots. So in the hypothetical it would go from 30 crimes to 17 crimes.

There won’t be a uniform formula to calculate the expected decrease in efficiency, but I think getting to perfect equality with the residential pop. will typically result in similar large decreases in many scenarios. A recent paper by George Mohler and company showed similar fairly steep declines. (That uses a totally different method, but I think will be pretty similar outputs in practice — can tune the penalty factor in a similar way to changing the linear program constraint I think.)

So basically the trade-off to get perfect equity will be steep, but I think the best case scenario is that a PD can say "this predictive policing strategy will not make current levels of DMC worse" by applying this algorithm on-top-of your predictive policing forecasts.

I will be presenting this work at ASC, so stop on by! Feedback always appreciated.

]]>Cory Haberman and Jerry Ratcliffe were the first I noticed to do this in this paper (Jerry’s near-repeat calculator has the option to export the strings). It is also a similar idea to what Davies and Marchione did in this paper.

Looking at these strings of events has clear utility for crime analysts, as they have a high probability of being linked to the same offender(s). Building off of some prior work, I wrote some python code to see what the distribution of these strings would look like when you randomly permuted the times in the data (which is the same approach used to estimate the intervals in the near repeat calculator). Here is the data and code, which is an analysis of 14,184 thefts from motor vehicles in Dallas that occurred in 2015.

So first I breakdown the total number of near repeat strings according to within 1000 feet and 7 days of each other. I then conduct 99 random permutations to see how many strings might happen by chance even if there is no near-repeat phenomenon. Some near-repeats can simply happen by chance, especially in places where crime is more prevalent. A length of string 1 in the table means it is not a near repeat, and 10+ means the string has 10 or more events in it. The numbers are the number of chains (in the Turchan article parlance), so 1,384 2-length chains means it includes 2,768 crime events.

If you compare the observed to the bounds in the table, you can see there are fewer isolates (1 length) in the observed than permutation distribution, and more 2 and 3 string events. After that the higher level strings occur just as frequently in the observed data than in the random data, with the exception of 10+ are *fewer*, but not by much.

So this provides evidence of the boost hypothesis in this data, albeit many near-repeat strings are still likely to occur just by chance, and the differences are not uber large. A crime analyst may be more interested in the question though "if I have X events in a near-repeat string, should I look into the data more". The idea being that since 2-strings are not that rare it would probably be a waste of an analysts time to dig into all of the two-events. I don’t think this is the perfect way to make that decision, but here is a breakdown of the distribution of strings for the permutated data.

So isolates happen in the random data 86% of the time. 2-strings happen 8.7% of the time, 3-strings 2.6%, etc. Based on this I would recommend that there needs to be at least 3 strings of near-repeat events if you have a low threshold in terms of "should I bother to dig into these events". If you want a high threshold though you may do more like 6+ events in a string.

This again is alittle bit of a slippage, as this is actual if you randomly picked a crime, what is the probability it is in a string of near-repeats of length N. I’m not quite sure of a better way to pose it though. Maybe it is better to think in terms of forecasts (eg given N prior crimes, what is the prob. of an additional near-repeat crime, similar to Piza and Carter). Or maybe in terms of if there are N near-repeats, what is the probability they will be linked to a common person (ala Mike Porter and crime linkage).

Also I should mention some of the cool work Liz Groff and Travis Taniguchi are doing on near-repeat work. I should probably just use their near-repeat code instead of rolling my own.

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