Many people would be happy for the scientist Omar Alyazeedi to take the credit for the downpour over Dubai this month. In these days when people grab the glory for the most feeble of achievements, it seems odd that Mr Alyazeedi is not trumpeting the triumph of his cloud-seeding project at the National Centre of Meteorology and Seismology.
Isn’t it obvious that the centre’s decade-long effort to persuade clouds to give up their rain has finally paid off? After all, the torrential rain – not exactly common here in August – followed his team putting salt crystals into the clouds.
Yet in interviews Mr Alyazeedi was keen to downplay such an inference. Instead, he emphasised the dangers of a fallacy that many people – and more than a few scientists – fall prey to.
Among logicians it is known as the post hoc, ergo propter hoc fallacy – from the Latin for “after this, therefore because of this”.
In other words, as the action that could influence an event happened before it, it must have been responsible for it.
Put like that, it seems hard to believe anyone could make such a blunder. Yet we all fall for it on a regular basis.
For example, we get a headache, so we take some aspirin for it … and sure enough we feel better. It seems like another demonstration of the power of medicine, but we are most likely fooling ourselves.
Studies of the effectiveness of aspirin have shown it works in only around 25 per cent of cases; the rest of the time, our pain would have subsided regardless.
The way to avoid the post hoc, ergo propter hoc fallacy is not to be overly impressed by how some effect was preceded by a plausible cause. It also helps to find out if the effect vanishes in the absence of its supposed cause. Medical scientists have a standard way of doing this, the randomised controlled trial.
They divide patients into two groups at random, giving those in the “treatment arm” the supposed cure for some ailment, while those in the “control arm” get some inactive placebo.
Inevitably, both groups will include people whose ailment would have subsided by itself, and others who would not respond to the treatment for a host of unknown reasons.
But all this is taken care of by the random way in which the patients were allotted to each group, which ensures that each has its fair share of such people. The only systematic difference between the groups is then whether or not they got the supposed cure.
Unfortunately, such comparisons are impossible in many situations – including analysis of this month’s cloud-seeding experiment.
Short of finding a comparison group of parallel universes where salt was not dumped in the clouds over Dubai, it is hard to see how to perform such analysis.
Mr Alyazeedi and his colleagues are hoping to do the next best thing, however, by creating a virtual “parallel universe” version of the UAE inside a supercomputer.
Working with scientists at the US National Centre for Atmospheric Research in Colorado, the team has developed simulations of the weather that can give clues to what might have happened had the clouds over Dubai not been seeded.
The simulation can be run many times with slight changes in the starting conditions, resulting in estimates of the statistical likelihood that the cloud seeding really was responsible.
It will be fascinating to see what they find. Perhaps they really have found a way of bringing summer rains to the UAE on demand.
Yet even supercomputers cannot help resolve most cases of the post hoc, ergo propter hoc fallacy.
Take the current furore in New York over its police department’s stop-and-search policy.
This was judged unconstitutional this month by a US district court judge, who assessed claims that certain ethnic groups were being specifically targeted by the police.
In her ruling, the judge, Shira Scheindlin, declared the policy to be a breach of the fourth amendment to the US constitution, which bans unreasonable searches and seizures, and of the 14th amendment, which gives all US citizens equal protection under the law, regardless of their ethnic origin.
The judgment has sparked a storm of controversy because stop-and-search has been hailed as a key part in the city’s crime-fighting campaign, during which murder rates have plunged to their lowest levels since the 1950s.
Many of those living in the Big Apple have been incensed by the judgment – among them Michael Bloomberg, the mayor of New York, who clearly sees it as an attack on part of his legacy to the city. He has said he would appeal against the decision.
Yet as a political science blogger, Prof Scott Lemieux, has pointed out, the claim that stop-and-search has caused the drop in crime carries a strong whiff of post hoc, ergo propter hoc.
Statistics show that crime rates in New York have been falling since the early 1990s – long before Mr Bloomberg’s tenancy of city hall.
But the mayor is far from alone in benefiting from public ignorance of the fallacy. Politicians across the G7 countries can point to a similarly precipitous fall in crime – even those who do not resort to New York Police Department-style policies.
The simplest explanation for this international trend is that there is no simple explanation.
As The Economist pointed out in its own analysis last month, everything from greater use of CCTV and anti-theft technology to the decline in use of crack cocaine probably has a role. And that explains why, even in this world of "evidence-based" policy, the post hoc, ergo propter hoc fallacy continues to thrive. It appeals to our craving for simple explanations to complex problems.
As children, we are exposed to countless examples of simple cause and effect. We touch something hot, we get burnt.
The trouble starts when we start applying the same simple rules to our complex world, or let politicians convince us they are responsible when something goes well.
We could all do worse than to follow the lead of Mr Alyazeedi, and be sceptical about our powers to change the world.
Robert Matthews is visiting reader in science at Aston University, Birmingham, England
