Just a few, initial thoughts: For starters, the 69% increase in marijuana cases in Austin from 2007 to 2010 is flat-out stunning, far dwarfing population growth, reported drug use trends, or any other conceivable, extraneous factor that might explain the data. That big a difference could only result from a policy change of some stripe. There certainly weren't that many more Austinites smoking pot in 2010 than in 2007.
Second, the drop in new marijuana cases from 2010 to 2011 may be explained by the radical reduction in traffic stops by Austin PD during the same period. "Austin police officers made 179,882 motor vehicle stops in 2011 compared to 232,848 in 2010," according to the city's racial profiling report. That's just a guess - I don't know what proportion of marijuana cases originate at traffic stops - but it seems like a logical correlation. Moreover, the previous years had witnessed higher than usual numbers of traffic stops thanks in part to now-dwindling federal grant funds that paid for overtime for that purpose. It's possible, in other words, that the number of marijuana cases is a dependent variable subject to the vicissitudes of the number of traffic stops overall. What other theories can readers suggest that might explain the data?
Finally, since my friend asked for the information broken out by race, it's worth noting that non-Hispanic white folks made up 34% of marijuana cases over this period compared to 48.7% of the population citywide in Austin, according to the 2010 census. Meanwhile, black folks make up just 8.1% of Austin's population, but a whopping 28.1% of petty marijuana cases. Latinos are the only one of the three largest ethnic categories who are arrested and cited for marijuana at roughly their proportion in the general population. Whether the disparity is driven by race or whether race is a proxy for other factors - like police deployment decisions - I cannot say. But I don't understand why the number of marijuana cases should be rising, and I certainly don't understand why black folks would be targeted so much more frequently for enforcement. Don't kid yourself. There are a LOT of white pot smokers in this town.
CLARIFICATION: Though the headline said "arrests," I conflated terms, The number of new marijuana "cases" increased 69%, but the data provided was both for arrests AND citations given for Class B and A marijuana possession. I apologize for any confusion.