The Washington Post covered a criminal justice story recently that tracked what happened to crime rates after communities changed the way welfare benefits are distributed. 

Previously, people received checks for public assistance. After they cashed these checks, they could have become targets of crime. Since the 1990's, however, public assistance benefits have gradually changed, and are now distributed via electronic debit cards (EBT cards). 

Did the shift to EBT cards cause a change in crime rate? Let's check it out. 

According to a team of researchers at the University of Missouri, the overall crime rate dropped 10 percent after the introduction of EBT cards for welfare. 

The graphic [in the story] looks at crime trends in Missouri before and after the switch from cash to debit cards, for all crimes and broken down by individual crimes. In most crime categories the change before and after the switch is striking – upward trends in assault, burglary, car theft and robbery are completely reversed.

a) What kind of quasi-experimental study is this? (Hint–looking at the graphs in the story will help you answer this question!)
Options: 
non-equivalent control groups pretest-posttest design
non-equivalent control groups posttest only design
interrupted time series design
non-equivalent control groups interrupted time series design

 

The journalist explains the following aspect of the results:

To put these results in perspective, the overall 10 percent decrease in crime corresponded to 47 fewer crimes per 100,000 people per county per month as a direct result of switching welfare benefits from cash to credit. This finding is fairly astonishing…. 

b) which big validity is the author addressing in the quote above?  

Here's another detail presented in the story. Rates of assault, burglary, car theft, larceny, and robbery all dropped after EBT cards, but rates of rape did not change. According to the authors, this result fits the theory that the availability of cash leads to less crime, because rapes are not typically about getting cash. 

c) Can the authors support the causal claim implied in the article's title: "To fight crime in your community, stop using cash"?

 

 

Suggested answers

a) this is an interrupted time-series design–it measured crime rates repeatedly up until some key event (the introduction of EBT cards), and repeatedly afterwards. 

b) By discussing the magnitude of the 10% increase in practical terms (47 fewer crimes per 100,000 people), the author is discussing one aspect of statistical validity (effect size). 

c) Let's apply the three causal criteria to this claim. There is definitely covariance–periods of time with EBT cards go with a lower crime rate, and periods of time with cash-based benefits go with a higher crime rate.

There is also temporal precedence. EBT cards came first, and crime rates were recorded afterwards.  

What about internal validity? We could work through the 12 major threats to internal validity here. The design and results of this study allow us to rule out selection, regression, maturation, and a variety of other internal validity threats. However, one possible culprit is a history threat–what else might have happened in the 1990's –at the same time that EBT cards were introduced–that might have also explained this drop in all crimes except for rape? Perhaps a change in policing strategy, political structure, or other crime policy occurred. By reading the scientists' original article, you might be able to check out how well the researchers addressed such history threats.  

 

Thanks to Carrie Smith for this study suggestion!