The Great Pacific Garbage Patch

Over lunch yesterday, in the middle of a conversation about global warming and wider environmental issues, a colleague told me she had read in the papers about a huge swathe of plastic garbage stretching across the Pacific Ocean. Sounded scary ! In case you haven’t heard of the GPGP before, here’s the Wikipedia entry

http://en.wikipedia.org/wiki/Great_Pacific_Garbage_Patch

Seems to have all the makings of an environmental horror story. Visions of mobile phones, plastic lunchboxes, barbie dolls, lego castles and what have you floating around in the middle of the ocean. Throw in the romance of this having been discovered by a guy out for a ride in his yacht, and it’s a made-for-TV environmental documentary.

Only there’s a couple of problems …

Firstly, most of the garbage can’t actually be seen because it has disintegrated into tiny particles. That’s not so romantic.

Secondly, and far more importantly, the numbers don’t add up. That is, the ones presented in the Wiki article at least.

It’s estimated that the GPGP contains 100 million tonnes of junk. That’s 10^11 kg.

Next, the area of the affected region is estimated to lie between 700,000 and 15 million sq. km. That’s a range of 7 x 10^11 to 1.5 x 10^13 sq. metres.

Hence, the density of garbage should lie between about 0.007 and 0.14 kg/m^2.

But in the section “Density of neustonic plastics” there is reference made to only one study, in which people took samples and found “concentrations of plastics at 3.34 pieces with a mean mass of 5.1 milligrams per square meter”.

So that makes 3.34 x 5.1 x 10^{-6} = 0.000017 kg/m^2.

That’s too small by a factor of anywhere between 400 and 8,000 compared to the first estimate.

Now Wikipedia is not a peer-reviewed journal, so maybe there’s a simple explanation for the discrepancy. And even if the problem is only 1/1000 as bad as the headlines suggest, then maybe it’s still significant … I don’t know. Maybe more samples need to be taken : if so, I want to be part of the expedition !

Correlation and causality

The title refers to one of the most common misues of statistics by people with a preset political agenda. An excellent example of this phenomenon appeared in the Swedish media yesterday evening when the lead headline on the Swedish national broadcaster’s text-TV service was the highly provocative “Fler kvinnliga lärare ger lägre löner” (More female teachers yields lower salaries).

http://svt.se/svt/jsp/Crosslink.jsp?d=22620&a=1472087&lid=senasteNytt_275214&lpos=rubrik_1472087

The article goes on to describe a study carried out by the Swedish Teacher’s Association (Lärarförbundet) which compares salary growth in the teaching profession over the last 50 years with that of engineers and business graduates. The chart shows clearly that teachers have performed poorly in this company with their average salary having gone from being 25% higher than in the other groups to being 25% lower today. It is noted that during the same period the percentage of female teachers has increased dramatically. The authors of the study thus conclude that teachers’ salaries are lower because the profession is female-dominated and that this is clear evidence of so-called “structural discrimination” in society.

Let’s assume for the sake of argument that the figures themselves in this study are accurate. The important point is that, even if this is so, the conclusions drawn are completely unwarranted. All that has been observed, to use the statistical parlance, is that two quantities

A = teachers’ salaries as a % of that of engineers/economists
B = % of female teachers

are negatively correlated, that is, A has decreased while B has increased. In general terms, there are three possible conclusions one can draw from such an observation (reality is usually somewhere in between) :

1. The decrease in A is caused by the increase in B
2. The increase in B is caused by the decrease in A
3. The two events are in fact entirely unrelated and are caused by other factors not present in the study. There might even be a common factor which causes both.

The study’s authors have opted for conclusion #1 : the headline on Lärarförbundets own website

http://www.lararforbundet.se/web/ws.nsf/documents/00364DCA?OpenDocument&menuid=00326A1C

makes it clear that SVT haven’t misinterpreted things. This is, at the very least, plain awful thinking. Conclusions 2 and 3 are just as consistent with the data and cannot be ruled out without further investigation.

Here’s an alternative hypothesis as to why teachers haven’t kept pace with other professions in Sweden in recent times. It could be because Swedish teachers are not as competent professionals as they used to be. A great dumbing-down of the educational system took place in Sweden during the 1970s which in particular involved a drastic reduction in the academic qualifications required of teachers. This process has slowed down in recent years but, unlike in some parts of the US, still has not gone into reverse. It’s a long story, and well known to everyone involved in education in Sweden. But as one concrete piece of evidence for this hypothesis, I give you this very study I’ve been talking about. If even the head of Lärarförbundet, Ms Eva-Lis Siren (who is quoted by SVT) is so academically incompetent as to not understand the difference between correlation and causality, then it doesn’t say much for her colleagues as a whole.

I’ll finish with a final piece of speculation. Lärarförbundet are openly using this study as an argument that teachers deserve to be paid better. So could it be that the whole point of the exercise is to open a negotiating front at a time when the economy is in crisis and cutbacks in public expenditures are being considered by all industrialised nations ?

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