For the next few weeks, I will be analyzing crime in Oakland using data from CrimeWatch. Please see my previous post on how I gathered and cleaned the data. My only rule is that I want everything I investigate to be actionable.
We know CrimeMapping under-reports recent crime by an average of 50% (where "recent" means the previous week). I wanted to see how this number varies by crime type. CrimeMapping claims that they only report closed cases. In other words, looking at when robbery reports begin dropping off will help us understand how much time passes before the OPD closes a case. Here are the crime reports by type, going back the last two months from the day I pulled the data:
This data was pulled on April 20, 2014. I plotted the last two months of crime reports up to that date, for both 2014 and 2013. For clarity's sake, I set up a 7-day moving average (every data point is the average number of reports for the previous week). Finally, I removed crime types that don't contain enough data (court visits, homicides, traffic, etc.) The 2013 data is there for comparison, the y-axis are the same for both years.
Each crime type's reporting declines the closer the data is to present day, but at different rates. Domestic violence cases are closed within the same week of being reported. Assaults and quality of life crimes take about a week to close. Larceny and vandalism are closed after 2-3 weeks of being reported.
Makes sense. Crimes that require investigation will take longer to close than crimes which don't. Domestic violence is a tragedy, but not a mystery. Misdemeanor assaults outpace felony assaults 2.4:1. And the vast majority of quality of life reports are narcotics.
What then about larceny? In 2013, the OPD had over 6,500 automobile burglaries, 6,000 stolen vehicles, 3,800 home burglaries, 3,700 robberies, and 3,600 petty thefts. This averages to about 70 reports a day. Oakland has the most robberies in America. That these cases appear in CrimeMapping means they're closed, but does not guarantee they are solved.
What is Oakland's robbery clearance rate? this SFGate article claims OPD had about 420 robbery arrests in 2011. According to city-data, they had about 3,300 robberies that year. This suggests Oakland has a 13% robbery clearance rate? That's less than half of the national numbers.
Let's put this all together: Assuming a 13% clearance rate, OPD solves just over 1 out of the 10 robbery reports they received each day in 2013. The remaining 9 are shelved in two weeks.