Correlations and causation

Image Credit: NASA, ESA, and the Hubble Heritage Team
Many people think that there have been a lot of big earthquakes in the past few months. In early January, a magnitude 7.1 earthquake hit the Solomon Islands, causing a small tsunami. One week later, a magnitude 6.5 earthquake hit just off the California coast. Two days after that, a magnitude 7.0 earthquake in Haiti killed hundreds of thousands of people. The Chilean magnitude 8.8 earthquake on February 27th was the 5th largest earthquake ever recorded. A few days ago, a magnitude 6.4 earthquake hit Taiwan, and today a magnitude 6.0 earthquake hit Turkey. (For a more detailed list of recent large earthquakes, see this list by the US Geological Survey.) Is there a relation between these earthquakes? Or are we just hypersensitized after the tragedy of Haiti? Or are we having the same frequency of big earthquakes as normal? More on this later in the post.

In astronomy, one of the common questions we ask is if two objects are related. In the picture at the top of this post, you see galaxies at the center of the galaxy cluster Abell S0740. Most of the galaxies you see are all ...