We haven’t had one of these for a while, but there’s some dodgy-looking extrapolation going on in the Keep New Zealand Beautiful litter audit. The audit itself is a good idea: measure litter in a detailed and reproducible way, so you can compare amounts now to amounts in the future and see whether things are getting greener and cleaner. And I don’t have any problems with how they conducted the survey.
But. The report (PDF) says (p18)
10,269,090,000 LITTERED CIGARETTE BUTTS polluting our ecosystem
and the Herald story says
Despite drops in smoking rates, discarded cigarette butts remained a big headache: some 10,269,090,000 were picked up, or 2,142 for every person in the country.
There weren’t 10 billion cigarette butts picked up. That would take a while. There were 39 cigarette butts picked up per 1000 square metres of land surveyed. With 10,000 477,000 square metres surveyed [update: I was confused by Table 2 in the report, which says 10,000 but is just illustrating the calculation (that’s the Table 2 on p25, not one of the others)], that comes to 390 18600. The detailed breakdowns in the report are fine, but there are also these extrapolations, where the amount of litter per 1000 square metres is scaled up by the number of 1000 square metre patches it would take to cover the whole country — about 260,000,000.
And, similarly, from the Herald quoting KNZB chief executive Heather Sanderson
“Extrapolated, that means 265,324,848 litres of illegal dumping – enough to fill 2,123 rail carriages, which if you stack them on top of each other, would be as high as 151 Sky Towers.”
is obtained by finding just under 1 litre (or 0.001 cubic metres) per 1000 square metres in the survey, and scaling up to the whole country. (Also, those imaginary rail carriages are being stacked end on end, which is probably not good for them)
Scaling up like this is how survey statistics works, but only if the sites you survey are an equal-probability random sample of the area The report doesn’t say they were, and it seems pretty unlikely, because it’s quite hard to get to a lot of randomly chosen bits of New Zealand, and these places — whether they’re up inaccessible mountains or in the middle of a big dairy farm — will tend to have less litter.
[Update: it’s obvious not an equal-probability sample of NZ; it could be some sort of stratified sample of the types of areas they were focusing on]
[Update, 13 September: Keep NZ Beautiful has modified the report to take out the dodgy extrapolations. Congratulations. The Herald hasn’t modified their story, though.]
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