Wednesday, 13 January 2010

Causality revisited

A while back I whinged that the press does a bad job of distinguishing correlation from causality. The latest headlines on correlations between TV-watching and mortality risk are no different: absent some kind of natural experiment or clever IV strategy, all we really know is that sedentary folks are more likely to die than folks who aren't.

For even more fun, check the shifting standards in the following paragraph:
Alcohol factors significantly in violent crime statistics: one third of all offences and half of all serious violent offences in NZ are committed by offenders who have been drinking (NZ Police); alcohol is associated with 46 percent of incidents of sexual violence (National Survey of Crime Victims, 2001). By contrast, nothing in the NZ Drug Statistics indicates cannabis use a cause of violent crime. Any association between cannabis and violence is linked to the black market and is therefore a direct result of prohibition itself.
So alcohol is bad because there may be correlations between drinking and criminal activity, but marijuana is good because nobody has established a causal relationship between weed and crime. As I'd previously noted:
...high proportions of arrests and crimes that are "related" to alcohol. But what I've never seen numbers on is what proportion those "alcohol related" crimes would have taken place in the absence of alcohol. You could just as easily say that 100% of crimes are "Oxygen related", 50% are mullet-related and 25% involve the wearing of t-shirts that say "No Fear". Ok, without the oxygen, none of those oxygen-related crimes would have taken place. But would a policy getting rid of No Fear t-shirts really prevent crime? Mightn't criminals switch to even riskier shirts? This is actually a reasonably serious point. If you ramp up the price of alcohol, how many criminals switch to P before going out to do nasty stuff? One of the country's more prominent criminologists suggests in informal chats that he reckons about 60% of these "alcohol-related" crimes would have taken place anyway even if alcohol disappeared; the actual crime-costs of alcohol are then about 60% lower than typically reported as alcohol in those cases is more like wearing a No Fear t-shirt: just something criminals like to do while offending but wouldn't stop them from offending in its absence.
I always find it depressing how folks like NORML think beating up on alcohol makes marijuana legalization more likely rather than just making for tighter regs. The hospitality industry lobbies for more restrictions on supermarket-bought alcohol to boost sales in bars; small brewers push for more punitive tax rates on big brewers... the only winners are the healthists who get support bit by bit for more regulations on everything. It's like a bunch of folks on the scaffolds complaining that the other guy's noose isn't quite tight enough. Y'all might instead direct your attention to the hangman sometime and try helping each other cut those ropes.