Analyzing Approaches to the Backlog of Untested Sexual Assault Kits in the U.S.A.

Abstract

Motivated by the debate over how to deal with the huge backlog of untested sexual assault kits in the U.S.A., we construct and analyze a mathematical model that predicts the expected number of hits (i.e., a new DNA profile matches a DNA sample in the criminal database) as a function of both the proportion of the backlog that is tested and whether the victim-offender relationship is used to prioritize the kits that are tested. Refining the results in Ref. (Criminol Public Policy, 2016, 15, 555), we use data from Detroit, where government funding was used to process %15% of their backlog, to predict that prioritizing stranger kits over nonstranger kits leads to only a small improvement in performance (a 0.034 increase in the normalized area under the curve of the hits vs. proportion of backlog tested curve). Two rough but conservative cost-benefit analyses-one for testing the entire backlog and a marginal one for testing kits from nonstranger assaults-suggest that testing all sexual assault kits in the backlog is quite cost-effective: for example, spending %$1641 to test a kit averts sexual assaults costing %$133,484 on average.

A cost-effectiveness analysis of the number of samples to collect and test from a sexual assault

“…14). We also note that the CODIS yield under full testing in Detroit, MI (3) was 723 1468 = 0.493 (5), and the CODIS yield in the Manhattan District Attorney's Office's Sexual Assault Kit backlog Elimination Grant Program, which allowed one random sample from a SAK to be tested, was less than 18,803 55,252 = 0.340, which is the mean number of CODIS entries per SAK. While these two quantities are not inconsistent with our findings, the CODIS yield in ref.…”

“…Moreover, the benefit-tocost ratio is somewhat similar for full testing (which is estimated to be 81.34 in ref. 5) and the SAFE policy (estimated to be 91.77), although the former policy more than doubles the CODIS yield; i.e., the additional effectiveness achieved by testing only samples deemed probative by SAFEs is mostly offset by the lack of economies of scale associated with testing so few samples per kit. Taken together, these results provide strong support for testing all samples in a SAK, as is currently done in the San Francisco Police Department Criminalistics Laboratory.…”

“…An alternative analysis could consider the number of CODIS hits (i.e., the number of CODIS uploads that match an existing DNA profile in CODIS) rather than the number of CODIS entries as the dependent variable of the machine-learning model. Other analyses suggest that the hit rate is higher for SAKs associated with stranger sexual assaults (and assaults that involve weapons) (3,5), which could lead to less aggressive testing of samples in nonstranger SAKs under the nonlinear priority policy (which allows the number of samples tested per SAK to vary) but not under the priority policy (which tests the same number of samples from each SAK). We note that if there is a CODIS entry but no CODIS hit in a nonstranger SAK, the entry may still deter the offender from committing future offenses (25).…”

“…Although this crime-solving approach appears to have considerable potential, >200,000 SAKs in the United States have been held in law enforcement storage facilities without ever being submitted to a forensic laboratory for DNA testing (1,2). Recently, several municipalities have tested their backlog of SAKs (3,4) and these undertakings appear to be cost-effective (4)(5)(6).…”

Comparison of Decision Tree and Logistic Regression Models for Utilization in Sexual Assault Kit Processing,

“…These findings from the validation study also hold true in a previous study that suggested a relationship did not exist between the number of years a sexual assault kit spent in evidence storage and the probability that the kit contained a probative DNA profile . Furthermore, a recent mathematical modeling of the untested backlog of SAKs in the United States found that testing stored SAKs was a cost‐effective approach to averting the overall cost of sexual assaults .…”