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Musings about the environment and all it touches, from education to city planning

Logical fallacies and the environment: hasty generalization and the ecological fallacy

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A sampling issue?  Each blind man should trade places...

A sampling issue? Each blind man should trade places…

As with the gambler and the sharpshooter fallacies, the ecological fallacy and hasty generalizations are two instances of poor statistical thinking (also known as inductive fallacies).

The nature of the hasty generalization fallacy can be guessed from its name: it is simply what results from arriving at a conclusion from too little data.  This is akin to concluding “all Germans have blue eyes” based on the five Germans that you happen to know.  It is also illustrated by the parable of the blind men and the elephant (each man should sample further!).

When doing environmental sampling, this simple fallacy can take many forms, difficult to detect.  For instance, saying that “based on our sampling, over half of the lakes in the province are polluted with hydrocarbons” may be a hasty conclusion.  Over half of the lakes sampled indeed showed high levels of hydrocarbons, above the limit.  But were ten lakes sampled? Or sixty?  This makes a big difference on whether the generalization is likely to be correct.

A particular problem with environmental sampling is what is called hotspots.  This is a typical issue when sampling to determine soil contamination.  For instance, imagine having to determine whether a site is contaminated.  The site used to be a 800 m2 minimall, including a gas station.  A gas tank had leaked below the station, polluting an area of eight square meters (the hotspot).  Ten samples taken at random are more likely than not to miss the hotspot; in this case a hasty conclusion would be to say that the site is not contaminated.

A close cousin of this fallacy is the artefact error, where a wrong conclusion may result from a biaised measurement system.  For instance, a fish survey claims that the average size of fish in a given lake is seven centimeters.  But the fishnet that was used to catch the measured fish has a mesh size of 0.5 cm, which means that none of the smaller fish that could pass through the net were included in the sample.  This type of error once led oceanographers to underestimate the number and variety of plankton since the very small varieties (now called nanoplankton, picoplankton, or even femtoplankton) were ignored as they were never noticed in the samples.

The ecological fallacy is the reverse of the hasty generalization.  It has nothing to do with ecology as we commonly use the term.  Whereas the generalization is faulty for extrapolating preliminary findings to a whole population, the ecological fallacy is at fault for assuming that all individual members of a population share its average characteristics.  In everyday language, stereotyping is an ecological fallacy.

Humans? a bunch of identical clones...

Humans? a bunch of identical clones…

An ecological fallacy occurs when one forgets, when studying a population of birds, say, that they are not all clones; variability is to be expected, both in physical traits and in behaviour.  Assuming that all birds will die from pollution after a toxic spill, for instance, may be faulty even if every bird is exposed to a dose that kills the average bird; some individuals may have a much higher tolerance to a toxin than others.  This error may also come even when variability is expected, if the type of variability is assumed wrongly.  If the average weight of a given bird population is 80 grams, so there must be as many birds lighter than 80 grams as there are heavier, right?  Wrong, if your sample consist of a hundred sparrows (30 grams each) and one five-kilogram Canada goose.  The mistake comes from assuming that the population has a normal distribution, that is, that it can be described by a bell-shaped curve.  (Using a bell-shaped curve to adjust student grades is often a faulty procedure, for the same reason.)

The website Fallacy-a-day has nice podcasts on these fallacies and many more.

 

Note: As instructors we’re expected to instill in our students critical thinking abilities. So this semester I included in my Environmental Issues class a review of some common logical fallacies with examples taken from the environmental scene. Much to my surprise, I couldn’t find such a collection through a Google search. So I started crafting my own by collecting examples of logical fallacies that occur in environmental news and discussions. Previous post in the series: the gambler and the sharp-shooter.

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Written by enviropaul

April 3, 2016 at 9:17 am

One Response

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  1. […] Note: As instructors we’re expected to instill in our students critical thinking abilities. So this semester I included in my Environmental Issues class a review of some common logical fallacies with examples taken from the environmental scene. Much to my surprise, I couldn’t find such a collection through a Google search. So I started crafting my own by collecting examples of logical fallacies that occur in environmental news and discussions. Previous post in the series: hasty generalization and the ecological fallacy. […]


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