Researchers are studying the relationship between the median salary of a police officer in a city and the number of violent crimes per 1,000 people. The researchers collected the police officers' median salary and the number of violent crimes per 1,000 people in 87 cities. After analyzing the data, researchers found that a city with a greater police officers' median salary tends to have a greater number of violent crimes per 1,000 people. The variables
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.

The greater police officers' median salary corresponds to a
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number of violent crimes per 1,000 people, so there is a
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correlation.

Causation is
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but it is unknown which variable causes the other. A greater median salary of police officers could cause a greater number of violent crimes per 1,000 people or a greater number of violent crimes per 1,000 people could cause a greater median salary of police officers. It is also possible that another factor causes a greater median salary and a greater number of violent crimes per 1,000 people.