Investigative Resources and Crime Clearances: A Group-Based Trajectory Approach

Abstract

Past studies of crime clearance rates have largely ignored the role of investigators in the process. This omission is important because criminal investigation is essential to clearing crimes, particularly those in which offenders are not readily identifiable. Using data from 570 law enforcement agencies spanning a 13-year period from 2000 to 2012, this study developed group-based trajectories of violent and property crime clearance rates (an approach not taken in previous clearance rate research), then modeled group membership with two measures of investigative spending and one of the proportion of investigators in sampled agencies (“investigative resources”), while controlling for factors such as workload and crime type. Results from a series of multinomial regression models suggest investigative resources play only a marginal role in crime clearance trajectories. This finding is consistent with early detective research, including the classic RAND criminal investigative process study.

Variability of Crime Clearance Among Police Agencies

“…If one's theory leads them to conclude that individuals or organizations have responded very differently to their environment over time, which has produced distinct patterns of change in an outcome, then GBTM provides an excellent way to simplify longitudinal differences and provide a more accurate representation of the distribution of an outcome. In our case, the use of trajectory modeling allowed us to demonstrate that agency clearance rates differed in marked ways from the national average rate over time and across crime type, suggesting that a group-based approach is a more accurate way to model clearance rates in future empirical work and discussion (see also Worrall, 2016). Compared with prior work, our analysis also allowed us to account for uncertainty in trajectory group membership, giving us more confidence in group assignments.…”

“…What is lacking from much research and theory on the dynamic relationship between clearance rates and other important variables, like investigative advances or crime rates, is an explicit statement regarding how agency clearance rate trends are distributed around the national average. One exception is a recent article by Worrall (2016). Worrall was one of the first to apply group-based trajectory modeling to the study of police clearances when he modeled variation in property and violent crime clearance trajectories from 2000 to 2012 for 570 law enforcement agencies.…”

“…Understanding variations in crime clearances is an important first step in determining what leads to improved clearance rates for agencies over time, and therefore is the first step of a larger project in which we are engaged with the Laura and John Arnold Foundation since 2015. Using similar techniques applied by Worrall (2016)—group-based trajectory modeling (GBTM; Nagin, 2005)—but on a more targeted group of agencies (very large agencies) and for a longer time series (33 years), we disaggregated violent and property crime types into specific offenses to analyze variations in clearance rate trajectories. We also add to previous work by applying a recent advancement in GBTM—multitrajectory modeling (Jones & Nagin, 2007; Nagin, Jones, Passos, & Tremblay, 2016)—to test our hypothesis that there may be identifiable groups of law enforcement agencies that have similar long-term clearance rate trends across multiple crime types, and that those group trends are distinct from the national average.…”

Clearing homicides

“…For example, Cloniger and Sartorius () concluded that small increases in enforcement efforts, as measured by expenditures and number of officers, did not affect clearance rates. More recently, Worrall (), using group‐based trajectory analysis, found a strong relationship between changes in agency spending per investigator and the long‐term clearance rate trends of an agency. Some researchers have estimated the “workload” of officers (and investigators) of agencies as a proxy measure for investigative effort (i.e., the greater the workload per officer, the less time spent on any given case).…”

The Impact of Investigation Strategies and Tactics on Homicide Clearance

“…For example, Eck (1992) found that clearance is often beyond the control of investigators and is an outcome based on incident event characteristics. Worrall's (2016) analyses of clearance rates for property and violent crimes by 570 law enforcement agencies over a 13-year period found that investigation tactics and resources have only marginal effects on clearance of violent crimes.…”