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<article xmlns:xlink="http://www.w3.org/1999/xlink">
  <front>
    <journal-meta>
      <journal-title-group>
        <journal-title>Bayesian Hierarchical Feature-Space Model Development for Long Term Data Study
Model Predictors Time Deviance MSPE
Spatial Choice and Trend</journal-title>
      </journal-title-group>
    </journal-meta>
    <article-meta>
      <title-group>
        <article-title>Investigating a Bayesian Hierarchical Framework for Feature-Space Modeling of Criminal Site-Selection Problems</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Jon Fox</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Samuel H. Huddleston</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Matthew Gerber</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Donald E. Brown</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <aff id="aff0">
          <label>0</label>
          <institution>Department of Systems and Information Engineering, University of Virginia 151 Engineer's Way Charlottesville</institution>
          ,
          <addr-line>Virginia 22904-4747</addr-line>
          ,
          <country country="US">USA</country>
        </aff>
      </contrib-group>
      <volume>35</volume>
      <issue>3049</issue>
      <abstract>
        <p>A significant amount of academic research in criminology focuses on spatial and temporal event analysis. Although several efforts have integrated spatial and temporal analyses, most previous work focuses on the space-time interaction and space-time clustering of criminal events. This research expands previous work in geostatistics and disease clustering by using a Bayesian hierarchical framework to model criminals' spatial-temporal preferences for site-selection across a continuous time horizon. The development of this Bayesian hierarchical feature-space model (BHFSM) offers law enforcement personnel a method for accurate crime event forecasting while improving insight into criminal site-selection at the strategic level. We compare the BHFSM to other featurespace modeling techniques using both a long range and short range criminal event dataset collected from police reporting. While the BHFSM remains sufficiently accurate for event prediction, current computational requirements limit the applicability for “just-in-time” crime modeling.</p>
      </abstract>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>Introduction</title>
      <p>
        Although much theoretical and practical work has been done
on the use of Bayesian hierarchical modeling for
geostatistics and disease clustering, applications within the
criminal site-selection problem have been limited. This article
merges the feature-space model of
        <xref ref-type="bibr" rid="ref30">Liu and Brown (1998)</xref>
        with the Markov random field construct of Zhu, Huang, and
Wu (2006) to model the criminal’s preference for initiating
a crime within a specific spatial-temporal zone. By adapting
theoretical and computational work from disease mapping
and environmental studies, we develop a Bayesian
hierarchical feature-space model (BHFSM) for the criminal event
prediction problem in order to examine both parameter
estimation and predictive inference. The remainder of this
section provides a quick review of applicable crime theory and
feature-space modeling for criminal site-selection problems.
The subsequent sections discuss the Bayesian hierarchical
framework, introduce the dataset used for this article, and
review the performance of the BHFSM against the dataset
for both a long term and a short term temporal study
horizon. In the final section, we review conclusions from this
initial research and propose paths for future research.
      </p>
      <sec id="sec-1-1">
        <title>Crime Theory</title>
        <p>
          Much of the work in crime studies proceeds from a frame
of reference built upon the location of the crime
          <xref ref-type="bibr" rid="ref1 ref23 ref43">(Townsley, Homel, and Chaseling 2000; Groff and LaVigne 2002)</xref>
          .
This frame of reference is conditioned upon Tobler’s first
law of geography: “everything is related to everything else,
but near things are more related than distant things”
          <xref ref-type="bibr" rid="ref42">(Tobler
1970)</xref>
          . For the crime analyst, this means that if a crime
happened yesterday at the corner of Main Street and Broadway,
then the most likely location for a crime tomorrow is the
corner of Main and Broadway. Hotspotting and crime
clustering are built upon the assumption that future crimes are
likely to occur at the same location as past crimes
          <xref ref-type="bibr" rid="ref10 ref17 ref28 ref36">(Ratcliffe
2004; Cohen, Gorr, and Olligschlaeger 2007)</xref>
          .
        </p>
        <p>
          Rational criminal theory assumes that individuals have
specific reasons for committing a crime at a certain time and
a certain location
          <xref ref-type="bibr" rid="ref9">(Clark 1980)</xref>
          . By examining the
historical criminal activity data within a spatial region, we can
discover patterns that might indicate criminals’ preferences
for executing crimes at certain locations
          <xref ref-type="bibr" rid="ref5">(Brantingham and
Brantingham 1984)</xref>
          . Spatial choice models offer analysts a
methodology for identifying a criminal’s preference for one
site over another within a spatial region.
        </p>
        <p>
          Spatial choice models assume an actor will select a site
(e.g., for migration, retail establishment, or criminal event)
based on the perceived utility, or worth, of that site from a set
of alternatives
          <xref ref-type="bibr" rid="ref16 ref33">(Ewing 1976; McFadden 1986)</xref>
          . The use of
spatial choice models nests well within the rational criminal
theory since it assumes that spatial point processes involving
actors are a result of the actors’ mental processes and
perceptions
          <xref ref-type="bibr" rid="ref8">(Burnett 1976)</xref>
          . This article expands on the spatial
choice problem to examine the impact of both geographic
and temporal features on the criminal’s site-selection
process.
        </p>
      </sec>
      <sec id="sec-1-2">
        <title>Feature-Space and Criminal Site-Selection</title>
        <p>
          This work is inspired by the following question: What if,
instead of focusing on where the crime happened on the
ground, we focus on where the crime initiation took place
in the mind of the criminal? The idea of spatial choice
presents a framework of decision processes for a rational
actor to choose a location based on the perceived value of
that location. Consider a criminal who wants to steal a car.
Will he choose a parking garage at the center of town with
restrictive traffic flows or will he choose the mall parking
lot near a major freeway on the outskirts of town?
Previous work has shown that the car thief will take the car
from the mall since features surrounding a location are as
critical as the location itself
          <xref ref-type="bibr" rid="ref37">(Rengert 1997)</xref>
          .
          <xref ref-type="bibr" rid="ref7">Brown, Liu,
and Xue (2001</xref>
          ) showed that data mining previous
criminal events provides insight to what spatial features might
be considered by a criminal in selecting a location to
commit a crime. We define this set of spatial considerations
to be the feature-space. Several investigations have shown
that feature-space modeling performs as well, or better, than
density based methods
          <xref ref-type="bibr" rid="ref40 ref40 ref6 ref6">(Brown, Dalton, and Holye 2004;
Smith and Brown 2004)</xref>
          .
        </p>
        <p>
          Criminal site-selection is the process by which a
criminal selects the time and space to execute an event based on
their feature-space preferences
          <xref ref-type="bibr" rid="ref35">(Porter 2006)</xref>
          . Rather than
using a latitude and longitude to describe each location in
a study region, we use spatial distances to environmental
features — such as schools, streets, or stadiums — and
spatial representations of social demographics — such as
population, percent rental properties, and household income
— to examine which locations are preferred by criminals
for certain types of crimes
          <xref ref-type="bibr" rid="ref1 ref1 ref2 ref24 ref31 ref31">(Bannatyne and Edwards 2003;
Liu and Brown 2003; Huddleston and Brown 2009)</xref>
          .
        </p>
      </sec>
    </sec>
    <sec id="sec-2">
      <title>Bayesian Hierarchical Modeling</title>
      <p>
        Hierarchical models allow us to deconstruct complex
problems into a series of smaller tractable problems. Using the
methodology developed by
        <xref ref-type="bibr" rid="ref44">Wickle (2003)</xref>
        , we formulate
the criminal site-selection problem into three basic stages:
a data model, a process model, and a parameter model.
Our data model accounts for our knowledge of the
spatialtemporal patterns of crime within the study region. The
process model provides insight to the criminal site-selection
process while accounting for spatial and temporal effects.
Finally, our parameter model accounts for the uncertainty in
both the data and process models
        <xref ref-type="bibr" rid="ref44">(Wickle 2003)</xref>
        .
      </p>
      <sec id="sec-2-1">
        <title>Formulation</title>
        <p>The goal of this article is to develop a Bayesian
hierarchical model that uses the feature-space methodology to
accurately predict crime events across an irregular lattice while
providing insight into the criminal site-selection process. To
estimate the criminal’s spatial preferences, our data model
represents the criminal’s site-selection process as a binary
random variable where Ys;t 2 0; 1 is the observation of the
presences, or absence, of crime at location s at time t given
a set of features X .</p>
        <p>Ys;tjX</p>
        <p>Bern( s;t)
(1)</p>
        <p>
          For our least complex model, we assume that the
probability s;t is a function of the criminal’s preferences for
certain features and a random effects term. Mathematically,
we represent the process model as:
kXsk +
s;t = logit 1 ( 0 + 1Xs1 + : : : +
for s = 1; :::; S and
for t = 1; :::; T :
(2)
Equation 2 uses a set of features X as a vector of length k for
each location s combined with the estimated values from
the parameter model to estimate the probability s;t. For
this article, we use a set of demographic variables to
represent a portion of the feature-space considered by the criminal
in their site-selection process. Analyzing previous criminal
event data gives us a method to account for the criminal’s
site-selection process. By modeling the relationship
between the features and the probability of crime, we estimate
the preferences criminals have for locations with a specific
set of features. However, just as the criminal’s preferences
for certain locations might change depending on
proximity to freeways or vacant houses, the criminal site-selection
process can also change depending on the time of day or
other seasonal events
          <xref ref-type="bibr" rid="ref20 ref21 ref29 ref39">(Rossmo, Laverty, and Moore 2005;
Gorr 2009a)</xref>
          . The variable s;t provides a method for
including other random effects.
        </p>
        <p>
          The first random effect considered is the temporal
component. We consider a temporal effect gt N (gt 1; g ).
Based on previous research
          <xref ref-type="bibr" rid="ref1 ref15 ref19 ref31">(Gorr, Olligschlaeger, and
Thompson 2003; Eck et al. 2005)</xref>
          , we believe that criminal
activity often preceeds criminal activity. Using this
temporal component allows us to account for periods of criminal
activity that match the routine activities and population
dynamics of the study region. We will discuss the inital
conditions for the variance estimates in the parameter model.
        </p>
        <p>
          We use a Markov random field (MRF) construct as the
second random effect by assuming that the likelihood of a
crime at a specific location is dependent only on its
neighbors and its previous temporal state
          <xref ref-type="bibr" rid="ref48">(Zhu, Huang, and Wu
2006)</xref>
          . Recent work on point processes uses MRFs as a
secondary structure that results from an aggregation process of
event counts. For our crime data, we construct the MRF
along an irregular lattice structure defined by political and
cultural boundaries using the construct provided by
          <xref ref-type="bibr" rid="ref25">Illian et
al. (2008)</xref>
          . We consider a MRF effect that accounts for the
past value at the location s and the second-order neighbors
such that !s N (!j 1; o). The index j accounts for the
second-order neighbors of location s. The inclusion of the
neighborhood spatial effects gives us a method to include
criminal repeat information into the feature-space model.
Studies on criminal repeats have shown that for short
temporal intervals, locations that have experienced crime have
an increased likelihood for repeat victimization
          <xref ref-type="bibr" rid="ref43">(Townsley,
Homel, and Chaseling 2000)</xref>
          .
        </p>
        <p>
          The third random effect considered for this article is an
interaction term. We consider an interaction term s;t
N (0; p). The interaction term is uncorrelated but can
identify potential spatial-temporal interactions within the data
that are not accounted for in the base feature-space model
          <xref ref-type="bibr" rid="ref29">(Lawson 2009)</xref>
          . The final random effect is an uncorrelated
error term vs N (0; tauv ) that accounts for any
uncorrelated spatial components of the criminal site-selection
process. The research design section outlines the four primary
models considered for this article using different
combinations of these random effects.
        </p>
        <p>Finally, we specify the parameter models by
establishing the initial distributions for the parameters. As seen in
Figure 1, the vector appears in the process model.
However, we provide initial estimates for the individual s within
the parameter model. Estimating the values increases the
complexity of the parameter model, since for both the long
term and short term data study, we initially estimate each
for each feature during the model fitting phase. In order
to reduce the computational requirements, we substitute a
feature-space prior calculated from linear model regression
(Lunn et al. 2000). The initial assumptions for the parameter
model follow:</p>
        <p>N ( ^; b)
N (0; svb); svb
N (0; svu); svu
N (0; svg); svg
N (0; svo); svo</p>
        <p>
          N (0; svp); svp
b
u (3)
g
o
p
The parameter model sets the initial conditions for the
simulation methods used to estimate the process and data model
and completes the model hierarchy
          <xref ref-type="bibr" rid="ref44">(Wickle 2003)</xref>
          . More
details on the simulation methods can be found in
          <xref ref-type="bibr" rid="ref27 ref29">(Lawson
2009; Kery 2010)</xref>
          .
        </p>
        <p>U (0; 10)
U (0; 10)
U (0; 10)
U (0; 10)
U (0; 10)</p>
        <p>
          Bayesian methods provide a means to calculate the
posterior distribution from our three stage-hierarchical model.
Using the example from
          <xref ref-type="bibr" rid="ref44">Wickle (2003)</xref>
          , our posterior
distribution is proportional to our data model conditioned upon
the process and parameter models times the process model
conditioned upon the parameters:
[process; parametersjdata] /
[datajprocess; parameters] (4)
[processjparametersl][parameters]
        </p>
        <p>
          Since our goals in modeling criminal site-selection
problems include both predictive inference and parameter
understanding, we desire to solve for the left hand side of
Equation 4. However, the complexity of the posterior distribution
makes obtaining a closed form solution almost, if not
completely, unobtainable. Using simulation methods, built upon
empirical knowledge from the data and expert knowledge on
the prior distributions, we obtain samples that provide
estimates of our target variables
          <xref ref-type="bibr" rid="ref29">(Lawson 2009)</xref>
          .
        </p>
      </sec>
      <sec id="sec-2-2">
        <title>Research design</title>
        <p>
          The Bayesian hierarchical feature-space model (BHFSM) is
a limited feature-space logistic regression model with an
auto-regression on the state of the neighboring locations
across an irregular lattice at discrete temporal intervals.
Following work from disease mapping and geostatistics, we
examine four models of random effects for our variable s;t.
The models considered provide several methods for
including other random effects
          <xref ref-type="bibr" rid="ref29">(Lawson 2009)</xref>
          . The four models
considered for random effects include:
        </p>
        <p>A time-varying trend gt plus an uncorrelated error vs
A Markov random field !s accounting for the sum of the
neighboring effects at a previous time plus vs
gt plus !s plus vs
gt plus !s plus vs and an interaction term
s;t</p>
        <p>Figure 1 displays a graphical representation of the third
model developed for this article without an interaction term.</p>
      </sec>
      <sec id="sec-2-3">
        <title>Study dataset</title>
        <p>The primary source of data for this article is an incident
database for the city of Charlottesville, Virginia. We
sample the complete dataset to develop a subset that contains a
time horizon spanning four years with over 2,000 incidents.
We restrict the crime types analyzed for this article to
assaults, both simple and aggravated. We drape an irregular
lattice over the study area and aggregate the criminal
incidents at the daily level. Although the aggregation introduces
some level of discreteness, we treat the temporal intervals as
continuous points along the temporal horizon. The
irregular lattice structure is based on the thirty-seven US Census
block-groups for the city. Using the this lattice structure
facilities inclusion of demographic information at the
blockgroup level. We use the census information as proxies for
complex factors that actually affect criminals. We are not
claiming that a criminal actually considers the percent of
houses in area that are rentals when deciding to execute a
crime. However, the percentage of rental houses in an area
might correlate with other factors that are part of the
criminal site-selection process. Figure 2 depicts the study region
draped with the irregular lattice and shows spatial-temporal
patterns of assaults over four distinct temporal intervals. The
analysis that follows uses a second-order neighbor structure
over the irregular lattice depicted in Figure 2.</p>
        <p>We set Yi;t = 1 if a criminal assault occurs within the
specified block-group i = 1; :::; 37 during one of the days
t = 1; :::; 1095 of the study horizon. The block-group and
daily aggregation results in a 37 365 matrix for a total of
40,515 observations in space-time. Figure 3 depicts a one
year snapshot of criminal events across the entire spatial
region.
For this article, we compare each model’s predictive
performance against a test set from the dataset. For the long
term study, we use a 365 day temporal window for model
fitting and then evaluate against a ninety day test. For the
short term study, we use a thirty day temporal window
surrounding special events in Charlottesville for model fitting
and then evaluate against the thirty day temporal window
surrounding the same special event in the following year.</p>
        <p>
          Prior to comparing predictive performance, we use a
goodness of fit measure to evaluate each model.
Borrowing from conventional generalized linear modeling, we use
deviance as a measure of how well the model fits the data. In
the software used for this article, we can expect the deviance
to decrease by 1 for each predictor added to the model
          <xref ref-type="bibr" rid="ref17 ref28">(Gelman and Hill 2007)</xref>
          .
        </p>
        <p>As an additional method for comparing goodness of fit,
we use the mean squared predictive error (MSPE). Given
our known spatial-temporal dataset from the test period, Y ,
our estimated spatial-temporal dataset, Y^ , and a number of
observations m from a simulation sample of G, we use
Lawson’s (2009) formulation such that:</p>
        <p>M SP E = j</p>
        <p>Y
(G</p>
        <p>Y^ j2
m)
(5)</p>
        <p>
          One of the challenges for spatial-temporal data is
selecting an appropriate statistical measure for examining model
performance. Originally used to assess radar performance
in World War II, the receiver operating characteristic (ROC)
curve are particularly useful for evaluating the ability of
a model to predict the occurrence of an event accurately
while minimizing the number of false positive predictions
          <xref ref-type="bibr" rid="ref4 ref41">(Bradley 1997; Swets, Dawes, and Monahan 2000)</xref>
          .
Similiar to the ROC curve, the surveillance plot provides a method
for evaluating model performance in spatial-temporal
classification problems. The surveillance plot gives the analyst
a method for monitoring the amount of area within the study
region that needed to be observed in order to identify the
highest percentage of crimes
          <xref ref-type="bibr" rid="ref17 ref2 ref24 ref28">(Huddleston and Brown 2009;
Kewley and Evangelista 2007)</xref>
          . Using a contingency table,
or decision matrix, similar to Table 1, we record the
possible outcomes of prediction estimated with the model being
considered against the true conditions observed in the test
set.
        </p>
        <p>
          We build the surveillance plot by plotting the rate of
accurate crime predictions against the rate of crime incidents
predicted where crimes did not occur. Although the
surveillance plot provides a measure for comparing model
performance visually, translating the surveillance plot into a
numerical measure provides a method for comparing the
performance of multiple models against a common test set. A
model with high accuracy — predicting all the crime
locations perfectly — would have a ratio of all true positives
versus zero false positives while a model with an equal
ratio of true positives and false positives is basically guessing
          <xref ref-type="bibr" rid="ref4 ref41">(Bradley 1997; Swets, Dawes, and Monahan 2000)</xref>
          .
        </p>
        <p>P LR =
n</p>
        <p>T P
(T P + F N )
(T P + F P )
(6)</p>
        <p>
          The performance limit ratio (PLR) measures the model’s
trade-off in accuracy and precision by focusing on the
model’s better-than-chance ratio
          <xref ref-type="bibr" rid="ref20 ref21 ref29">(Gorr 2009b)</xref>
          of correctly
predicting crimes within a test set of size n. A model that
is more accurate in predicting crimes across the space-time
surface will have a higher PLR. Rather than focusing on the
entire area under the curve, we reduce the focus to the first
20% of the space-time surface observed while discounting
the area under the curve that accounts for random guessing.
        </p>
      </sec>
    </sec>
    <sec id="sec-3">
      <title>Long Term Study Results</title>
      <p>
        For the long term study, the block-group and daily
aggregation results in a 37 365 matrix for a total of 13,505
observations in space-time. We use a second-order neighbor model
to account for all the criminal activity in all the
surrounding census blocks. Table 2 outlines the specific models
examined using both the demographic features and a
featurespace prior obtained from a generalized linear regression
similar to the work of
        <xref ref-type="bibr" rid="ref1 ref31">(Liu and Brown 2003)</xref>
        . As discussed
in the research design section, we consider four alternatives
to model the random effects using our variable s;t in the
process model: 1) a time-varying trend; 2) a Markov
random field accounting for the sum of the neighboring effects
at a previous time; 3) a time-varying trend with a Markov
random field; 4) a time-varying trend with a Markov
random field and an interaction term. For every alternative we
include a space-time independent noise term. For the first
four alternatives, we attempt to account for the criminal
siteselection preference by modeling as seen in Figure 1.
After model fitting, we evaluate performance using the MSPE
discussed above.
      </p>
      <p>
        Although the predictive performance of the BHFSM is
not significantly better than the base feature-space model,
we were expecting to see significant lift in the parameter
estimation related to identifying criminal preferences for
certain spatial features. In fact, even with all four
models converging, the only feature-space variable with
significantly better estimation was the preference for areas with
high percent vacancy. However,
        <xref ref-type="bibr" rid="ref29">(Lawson 2009)</xref>
        shows that
the combination of spatially-referenced explanatory
variables within a Markov random field construct often yields
poor estimates of the regression coefficients and produces
computational challenges related to multi-collinearity. Both
of our approaches to reduce the impact of correlation
created additional challenges. First, removing the features that
are spatially dependent limits our insight into the criminal
site-selection process for identifying feature-space
preferences. Second, introducing new variables that have a
stationary spatial attribute but are non-stationary temporally
limits our ability to identify how the criminal’s feature-space
preferences evolve over time. Overall, the Bayesian
approach offers promise for reducing uncertainty in the
predictive surfaces. However, as discussed in
        <xref ref-type="bibr" rid="ref45 ref46">(Withers 2002;
Zhu et al. 2008)</xref>
        , the computational time required for
sampling from the posterior distribution for Bayesian inference
for criminal site-selection problems is a major drawback.
We discuss an alternative approach in the conclusion that
offers computational advantages while remaining sufficiently
accurate for prediction. In the next section, we scale down
the horizon of the study period as an additional step in
examining the BHFSM.
      </p>
    </sec>
    <sec id="sec-4">
      <title>Short Term Study Results</title>
      <p>
        Although applying the Bayesian framework to the long term
study data did not result in significant gains in predictive
performance, the initial disappointment was not entirely
unexpected. Previous research shows that spatial-temporal
analysis focused on criminal site-selection requires focused
efforts on periods of temporal transition and local knowledge
of the environment
        <xref ref-type="bibr" rid="ref2 ref24 ref26">(Kerchner 2000; Bernasco and Block
2009)</xref>
        . A more appropriate methodology for including
temporal information into the BHFSM reduces the scope of the
temporal horizon to those intervals with the greatest variance
in crime rates. Research has also shown that spatial regions
experience great variance in crime rates for certain locations
depending on the temporal proximity to special events
        <xref ref-type="bibr" rid="ref10 ref17 ref28">(Cohen, Gorr, and Olligschlaeger 2007)</xref>
        . Reducing the
temporal horizon to a smaller scale — such as a thirty day
window before and after large spikes in crime rates — makes it
easier to examine the impact of these special events on the
criminal site-selection process. More importantly, including
additional data from local law enforcement personnel takes
advantage of their local knowledge of the temporal
environment
        <xref ref-type="bibr" rid="ref11">(Cressie and Wikle 2011)</xref>
        .
      </p>
      <p>
        As with the long term study, we consider all four
alternatives to model the random effects using our variable s;t in
the process model. Table 3 outlines the specific models
exModel
Feature-Space Model
Spatial Choice and Trend
Spatial Choice and MRF
Spatial Choice and MRF and Trend
Spatial Choice and MRF and Trend and Interaction
Feature-Space Prior and Trend
Feature-Space Prior and MRF
Feature-Space Prior and MRF and Trend
Feature-Space Prior and MRF and Trend and Interaction
amined using both the demographic features and a
featurespace prior obtained from a generalized linear regression
similar to the work of
        <xref ref-type="bibr" rid="ref1 ref31">(Liu and Brown 2003)</xref>
        and as seen
in our visual graph from Figure 1. Again, the only
featurespace variable with significantly better estimation was the
preference for areas with high percent vacancy. After model
fitting, we evaluate performance using the PLR discussed
above. While each BHFSM performs better than the base
feature-space model, the computational time required for
sampling from the posterior distribution for Bayesian
inference is still several orders of magnitude greater than the time
required for using generalized linear regression on the base
feature-space model.
      </p>
    </sec>
    <sec id="sec-5">
      <title>Conclusions</title>
      <p>For city-wide, or regional-level, crime monitoring, the
BHFSM offers a methodology for modeling criminal
activity across continuous time. For this article, we added a
Bayesian framework to the base feature-space model to
include variables that account for both spatial and temporal
patterns within the criminal site-selection process. We
applied this methodology to both a long term and short term
data study for criminal events in a small US city. Using data
aggregated at the census block-group level for a medium
temporal resolution, the BHFSM allowed us to model an
actor’s spatial-temporal preferences within a limited
temporal period. Incorporating elements of the feature-space
methodology into the Bayesian construct allowed us to blend
the benefits gained from understanding multiple covariates
within the actor’s spatial-temporal decision process with the
basic elements of geographic recency and spatial
dependence found in hotspot modeling. Although the overall
predictive performance is not significantly improved, by
reducing the variance on estimates for a criminal’s feature-space
preferences, we gain understanding into the temporal
variations of the criminal site-selection process. Enhanced
understanding of the criminal site-selection process allows law
enforcement personnel to adjust resource allocation
strategies to better mitigate short term changes in the criminal
site-selection process.</p>
      <p>
        Several challenges remain for further consideration of
the Bayesian framework for feature-space modeling of the
criminal’s site-selection process. The methodology
examined in this article is computationally intensive. Although
the BHFSM did provide improvement in predictive
performance over the base feature-space model for the short term
data study, the increased computational requirements hinder
the application of the BHFSM for “just-in-time” crime
modeling. Extending the Bayesian framework for modeling data
at either a finer temporal or spatial resolution would increase
the computational complexity since the size of the
spatialtemporal event matrix is a multiple of the temporal intervals
and the spatial dimensions. Future work will attempt to
reduce this computational complexity by adding temporal and
neighborhood indicator functions to the base feature-space
model
        <xref ref-type="bibr" rid="ref12 ref30">(Diggle, Tawn, and Moyeed 1998)</xref>
        . Using indicator
functions allows for faster sampling from the data while still
accounting for temporal preferences in the criminal’s
siteselection process.
      </p>
      <p>
        Structural vector autoregressive models (SVARs) show
promise for forecasting employment rates given spatially
based economic indicators
        <xref ref-type="bibr" rid="ref2 ref24 ref38">(Rickman, Miller, and
McKenzie 2009)</xref>
        . Using an SVAR construct for modeling criminal
site-selection might improve predictive ability if temporal
changes in other features affect a criminal’s temporal
considerations for certain sites. However, the computational
requirements for SVARs, like the Bayesian construct, are still
rather demanding
        <xref ref-type="bibr" rid="ref2 ref24 ref34">(Petris, Petrone, and Campagnoli 2009)</xref>
        .
      </p>
      <p>
        The social sciences offer another approach for reducing
the computational demands of criminal site-selection
modeling. Spatial-temporal designs for environmental research
often include panel methods for monitoring and detecting
temporal patterns and spatial relationships
        <xref ref-type="bibr" rid="ref13">(Dobbie,
Henderson, and Stevens 2008)</xref>
        . We are not designing a method for
collecting criminal event data, but rather examining
historical collections of crime data. And as mentioned above,
studies at fine temporal and spatial resolutions require a large
spatial-temporal event matrix. Using a variation of stratified
sampling
        <xref ref-type="bibr" rid="ref13 ref18">(Gilbert 1987; Dobbie, Henderson, and Stevens
2008)</xref>
        on the spatial-temporal event matrix might reduce the
computational time while retaining comparable predictive
performance.
      </p>
    </sec>
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