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Sex offender boundary restrictions:A geospatial approach
Michael ChajewskiFordham University
NIJ July 2008
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Conceptual Framework
• Wetterling Act (1994): Mandatory Offender registration.
• Megan’s Law (1996): Community Notification.
• As of 2007 twenty-seven states had enacted some form of residency restrictions.
• Both state and local jurisdictions utilize residency restrictions typically ranging from 500 to 2,500 feet.
• Data can be complied from various sources (GPS tracking devices, Offender Registries etc.) and manipulated in several software packages (arcGIS, GRASS, GeoDa, R).
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Territory
500 feet restriction
School
2500 feet restriction
Sex offendersnot in violation
Sex offendersin violation
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Restricted Area
r
• r (radius) = 500 feet
• Area of circle = π r 2
• Area of a single 500 feet buffer = 785,398 feet2 (or 0.0282 miles2)
• Area of a single 2500 feet buffer = 19,634,954 feet2 (or 0.7043 miles2)
• Buffer zones, however, overlap and create larger polygons.
School
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Territory
School
Calculated Distances
Streets
Sex Offender
Comparison
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WebbElementary School
Webb Elementary School1375 Mount Olivet Rd NEWashington, DC 20002
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??
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An Example
• Newark, NJ is generally included in analyses pertaining to the greater metropolitan area of New York City (mainly because of its accessibility and international airport).
• The population is roughly 273,546 people.
• There are 118 schools (K-12) in Newark, NJ (Essex County).
• As of June, 2006 there were 196 registered sex offenders within the city limits.
• A comparison set of 196 randomly selected residential addresses from the area have been selected.
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Newark, Essex County, New Jersey
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Newark, Essex County, New Jersey
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In practice
• Boundary restrictions ought to apply to the population for which they are intended.
• Based on work by Hanson and Morton-Bourgon (2005) on average 13.7% of the sex offenders recidivated within the first 5 years. Recidivism impact cannot be evaluated and/or predicted based on residential geographic information alone.
• Residency restrictions as an alternative to monitor, control and supervise the sex offender population, as envisioned by policy makers, is extremely costly, labor intensive and invasive.
• Geospatial models used need to be carefully selected, justified, and analyzed (including error calculations, alternative approaches, plausible explanations, missing data evaluation; correlation is NOT causation).
• Other research looking at boundary restrictions: Julie Wartell (2007), Nobles et al. (2007), Grubesci et al. (2007; 2008).