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# Correlation and Causation

## Correlation means that two trends go together; causation means that one of those trends causes the other.

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Correlation and Causation

Sugar consumption up. Global temperatures up. Is one causing the other?

Is the rise in sugar consumption in America causing average global temperature to rise? Are rising temperatures causing people to eat more sugar? Both of these factors are rising but are they related by correlation, causation, or both?

### Vocabulary

• causation: The relationship between an event and another event in which one event caused the other event.
• correlation: A relationship between two or more things, but not a cause-and-effect relationship.
• mechanism: The process by which something takes place.
• negative correlation: A relationship between two things in which one increases and the other decreases.
• positive correlation: A relationship between two things in which both change in the same direction, either increase or decrease.

### Correlation

We made a few discoveries in the previous sections:

• Average global temperature has been rising for the past several decades.
• Atmospheric carbon dioxide levels have been rising for the past several decades.
• Carbon dioxide emissions into the atmosphere from fossil fuel burning have been rising for the past several decades.

We see a correlation. A correlation is the mutual relationship between two or more things. CO2 emissions from fossil fuel burning, atmospheric CO2 levels, and average global temperatures are all rising. They exhibit positive correlation because they are all going in the same direction. If one factor rises while another sinks they have negative correlation.

### Causation

But correlation does not necessarily indicate causation. To explain the difference, let’s look at an example. Sugar consumption in the United States has also been rising for decades. This is positively correlated with rising average global temperatures.

American consumption of caloric sweeteners (sugar), 1970-2009.

Is the rise in sugar consumption causing the rise in global temperatures or vice versa? While this isn't impossible; it's extremely unlikely. There’s no mechanism for one to increase the other. Here there is correlation, not causation.

Causation refers to the factor that is producing the effect. We sometimes refer to this as a cause-and-effect relationship.  To establish causation we need to know the mechanism, or how one would cause the other.

Here is a brief outline of the way an increase in CO2 can increase global temperatures. (Climate change science is dealt with extensively in the "Climate Change" concepts.)

• Greenhouse gases in the atmosphere trap heat. This is natural and good.
• CO2 is a greenhouse gas.
• The more greenhouse gases there are in the atmosphere, the more heat can be trapped.
• The more heat that’s trapped, the warmer average global temperatures are.

Because carbon dioxide is a greenhouse gas, increased atmospheric CO2 causes average global temperatures to rise. We found one cause for rising global temperatures. There are also others.

### Summary

• Correlation is a comparison of two factors. Correlation is not the same as causation.
• If one factor is responsible for the change in another factor, there is causation.
• Establishing causation requires a mechanism to show how one factor can influence the other.
• The purpose of science is often to discover the mechanism of causation between two or more things.

### Practice

Use these resources to answer the questions that follow.

Correlation vs. Causality: Freakonomics Movie (http://www.youtube.com/watch?v=lbODqslc4Tg)

1. Why did people fear polio?

2. Why did people think there was a correlation between ice cream and polio?

3. Why are there more police in Washington DC than in Denver? Where is there more crime? Do police cause crime? Then why are there more police in Washington DC?

4. Explain why people often confuse correlation and causation.

### Review

1. Compare and contrast correlation and causation.

2. Sugar consumption has been rising in the U.S. for decades. Can you think of something this might be positively correlated with? Can you think of something this might be negatively correlated with?

3. Name at least two factors that are changing as a result of the increase in sugar consumption in the U.S. Is this correlation or causation?

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