How an anecdote becomes a statistic (based on 'research')

Everyone has heard about those astonishing facts whose source proves tricky to uncover.  “We only use 10% of our brain” is one example, usually followed by some encouragement to undertake a course in new ways of thinking.  There is no scientific basis for this supposed fact.

The same is true of statistics, when numbers are touted around supposedly supporting an argument (or perhaps prejudice) but whose origin proves elusive.  Here, for once, is a nice example where we can trace the story back to its source, and discover how tenuous it is. 

The Daily Telegraph reported on October 31^st 2011 (see here) that there are more Porsches in Greece than there are taxpayers declaring an income above 50,000 Euros per annum, “according to research by Professor Herakles Polemarchakis, former head of the Greek prime minister’s economic department”.   This suggests support for the assertion that tax evasion is widespread in Greece, that therefore the fiscal crisis is their own fault and hence that austerity measures can be justified.

The story is interesting for several reasons:

1.      A simple anecdote is preferred to statistical evidence,  

2.      For once, there is a reported source of the story, and

3.      There is some confusion about whether the story relates to all Porsches or the Cayenne model in particular (different news agencies have different versions).

The BBC followed up this story on the 16^th April 2012 (so the story had been going unchallenged for at least six months) with some fact checking, see here for details.  First they found that there were 311,428 people with incomes above 50,000 Euros in 2010.  Then they asked Porsche how many Cayennes they sold in Greece.  Their spokesman (after he finished laughing) said they had sold around 1,500 over the previous nine years. 

So what of Professor Polemarchakis’ research?  When asked, he replied that his remark was casual, based on what had been circulating in policy circles in Greece a few years back.  He said the only hard fact he was aware of was "the per capita number of Cayennes in [the Greek city of] Larissa was twice that of Cayennes in the OECD countries".

So it is that a casual observation becomes an anecdote which becomes evidence based on research.  However, it is one of those stories where one should immediately be suspicious of the “fact”.  A quick Google check shows Porsche’s European sales in 2011 were 42,084, so Greece is unlikely to take more than (at a guess) 10% of them, around 4,000 vehicles.  To have over 300,000 Porsches (never mind the Cayenne model only) in the country means they must have amassed them over 75 years…