Overcounting causes

There’s a long story in the Sunday Star-Times about a 2007 report on cannabis from the National Drug Intelligence Bureau (NDIB)

“Perhaps surprisingly,” Maxwell wrote, “cannabis related hospital admissions between 2001 and 2005 exceeded admissions for opiates, amphetamines and cocaine combined”, with about 2000 people a year ending up in hospital because of the drug.

The problem was with hospital diagnostic codes. Discharge summaries include both the primary cause of admission and a lot of other things to be noted. That’s a good thing — you want to know what all was wrong with a patient both for future clinical care and for research and quality control.  For example, if someone is in hospital for bleeding, you want to know they were on warfarin (which is why the bleeding happened), and perhaps why they were on warfarin. It’s not even always the case that the primary cause is the primary cause — if someone has Parkinson’s Disease and is admitted with pneumonia as a complication, which one should be listed? This is a difficult and complex field, and is even slightly less boring than it sounds.

As a result, if you just count up all the discharge summaries where ‘cannabis dependence’ was somewhere on the laundry list of codes, you’re going to get a lot of people who smoke pot but are in hospital for some completely different reason.  And since there’s a lot of cannabis consumption out there, you will get a lot of these false positives.

There are some other things to note about this report, though. The National Drug Foundation says (on Twitter) that they made the same point when it first came out. They also claim

@SundayStarTimes In fact the health ministry recommended NDIB not publish it because it was so poorly done 2/2 (we have OIA from back then)

— NZ Drug Foundation (@nzdrug) April 16, 2016

that the Ministry of Health argued against its being published.

Perhaps now the multiple-counting problem has been publicised in the context of hospital admissions the same mistake will be made less often for road crashes, where multiple factors from foreign drivers to speed to alcohol to drugs are repeatedly counted up as ‘the’ cause of any crash where they are present.