Bad Science

cent. (2003–05 data in table under references).

The rising trend of cannabis potency is gradual, fairly unspectacular, and driven largely by the increased availability of domestic, intensively grown indoor herbal cannabis.

Mean potency (% THC) of cannabis products examined in the UK (Forensic Science Service, 1995–2002)

Year Sinsemilla %Resin %‘Traditional’ imported herbal %

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

5.8

8.0

9.4

10.5

10.6

12.2

12.3

12.3

12.0

12.7

14.2

No dada

No dada

No dada

6.1

4.4

4.2

6.7

3.2

4.6

1.6

6.6

3.9

5.0

4.0

3.8

5.0

8.5

No dada

No dada

No dada

No dada

No dada

Mean THC content of cannabis products seized in the UK (Forensic Science Service, 1995–2002)

‘Twenty-five times stronger’, remember. Repeatedly, and on the front page.

If you were in the mood to quibble with the Independent’s moral and political reasoning, as well as its evident and shameless venality, you could argue that intensive indoor cultivation of a plant which grows perfectly well outdoors is the cannabis industry’s reaction to the product’s illegality itself. It is dangerous to import cannabis in large amounts. It is dangerous to be caught growing a field of it. So it makes more sense to grow it intensively indoors, using expensive real estate, but producing a more concentrated drug. More concentrated drugs products are, after all, a natural consequence of illegality. You can’t buy coca leaves in Peckham, although you can buy crack.

There is, of course, exceptionally strong cannabis to be found in some parts of the British market today, but then there always has been. To get its scare figure, the Independent can only have compared the worst cannabis from the past with the best cannabis of today. It’s an absurd thing to do, and moreover you could have cooked the books in exactly the same way thirty years ago if you’d wanted: the figures for individual samples are available, and in 1975 the weakest herbal cannabis analysed was 0.2 per cent THC, while in 1978 the strongest herbal cannabis was 12 per cent. By these figures, in just three years herbal cannabis became ‘sixty times stronger’.

And this scare isn’t even new. In the mid-1980s, during Ronald Reagan’s ‘war on drugs’ and Zammo’s ‘Just say no’ campaign on Grange Hill, American campaigners were claiming that cannabis was fourteen times stronger than in 1970. Which sets you thinking. If it was fourteen times stronger in 1986 than in 1970, and it’s twenty-five times stronger today than at the beginning of the 1990s, does that mean it’s now 350 times stronger than in 1970?

That’s not even a crystal in a plant pot. It’s impossible. It would require more THC to be present in the plant than the total volume of space taken up by the plant itself. It would require matter to be condensed into super- dense quark-gluon-plasma cannabis. For God’s sake don’t tell the Independent such a thing is possible. Cocaine floods the playground

We are now ready to move on to some more interesting statistical issues, with another story from an emotive area, an article in The Times in March 2006 headed: ‘Cocaine Floods the Playground’. ‘Use of the addictive drug by children doubles in a year,’ said the subheading. Was this true?

If you read the press release for the government survey on which the story is based, it reports ‘almost no change in patterns of drug use, drinking or smoking since 2000’. But this was a government press release, and journalists are paid to investigate: perhaps the press release was hiding something, to cover up for government failures. The Telegraph also ran the ‘cocaine use doubles’ story, and so did the Mirror. Did the journalists find the news themselves, buried in the report?

You can download the full document online. It’s a survey of 9,000 children, aged eleven to fifteen, in 305 schools. The three-page summary said, again, that there was no change in prevalence of drug use. If you look at the full report you will find the raw data tables: when asked whether they had used cocaine in the past year, 1 per cent said yes in 2004, and 2 per cent said yes in 2005.

So the newspapers were right: it doubled? No. Almost all the figures given were 1 per cent or 2 per cent. They’d all been rounded off. Civil servants are very helpful when you ring them up. The actual figures were 1.4 per cent for 2004, and 1.9 per cent for 2005, not 1 per cent and 2 per cent. So cocaine use hadn’t doubled at all. But people were still eager to defend this story: cocaine use, after all, had increased, yes?

No. What we now have is a relative risk increase of 35.7 per cent, or an absolute risk increase of 0.5 per cent. Using the real numbers, out of 9,000 kids we have about forty-five more saying ‘Yes’ to the question ‘Did you take cocaine in the past year?’

Presented with a small increase like this, you have to think: is it statistically significant? I did the maths, and the answer is yes, it is, in that you get a p-value of less than 0.05. What does ‘statistically significant’ mean? It’s just a way of expressing the likelihood that the result you got was attributable merely to chance. Sometimes you might throw ‘heads’ five times in a row, with a completely normal coin, especially if you kept tossing it for long enough. Imagine a jar of 980 blue marbles, and twenty red ones, all mixed up: every now and then – albeit rarely –