2014 The Bad Science Show - Melbourne School Shows & School Incursions
Water Divining (Correlation Vs Causation
“Has anyone else seen those incredibly powerful advertisements in cinemas where each time a famous person clicked their fingers, an African child dies?
"I watched those, and couldn’t help thinking, ‘Well stop clicking your fingers!'”
Jimmy Carr
*This resource is intended for use after viewing The Bad Science Show. Students should be reminded of the portion of the show the lesson relates to.
Purpose
To learn about the difference between causation and correlation using the example of water divining from The Bad Science Show. Students should understand what causation and correlation are and be better equiped to spot them in the real world.
Introduction
Water Divining is the belief that, by waving a rod, pendulum or wooden stick over the ground and waiting for it to move, water can be found. Diviners will walk around waiting for their divining tool to move. When it does, they dig and find water!
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This is an example of correlation being confused with causation.
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If there is a correlation between two variables the events are connected but one does not directly cause the other to happen.
This may because of pure chance. E.g. I won that hot dog eating contest while eating my lucky undies. Therefore, my lucky undies made me win.
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However it may also be that the two variables are caused by a third variable. E.g. Drowning rates go up as ice cream sales go up. This not because ice cream causes drowning but because more people swim and eat ice cream in hot weather.
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If there is causation then it means one variable caused the other. e.g. I get more fat the more hot dog eating competitions I enter.
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Water Diviners believe that a pendulum, wooden stick or rod moves because of water under the ground. They believe that the water causes the rod to move. In reality, there is not casual relationship between the two events. The divining tool moves because their hands move and the water is found because there is a huge amount of water under the ground. In other words, pure luck.
Discussions
1) What is the cause of the divining rod moving? (the diviners hands are moving) What is the cause of water being found? (there is a lot of water to be found)
2) How would a water diviner explain NOT finding water where it is supposed to be?
3) How would you prove that there is no causal link between the rods moving and water being found?
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Activities
In groups, have the students create their own psuedoscience by passing off correlation as causation.
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1) Have the students list two variables that have the same cause.
e.g.
Cause Variable One Variable One
Baldness Using Less Shampoo Less Hair Combing
Hot Weather Higher Ice Cream Sales More Sunburn
Windy Days Trees Move Clouds Blow Away
2) Remove the actual cause
e.g. baldness, hot weather and windy days.
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3) Falsely draw a connection between the two variables.
e.g. Using less shampoo makes your hair need less combining!
Eating too much ice cream leads to your skin turning red!
Trees wave back and forth and fan away clouds!
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Have the class discuss which of the spurious correlations was the most convincing? Why?
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After The Class
The website Spurious Correlation regularly posts silly graphs showing the correlation between two random events. For example, did you know that the more films Nicolas Cage appears in each year, the higher the death by drowning rate is? Nicolas Cage movies cause drownings!
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Further Reading​
Spurious Correlations By Tyler Vigen
The Mighty Mitta Muster Water Diving Test by Australian Skeptics
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Nicholas J. Johnson
2017