Do a chart with two variables to measure the degree of correlation between them, It is probably the most used statistical tools. And like any other analysis, We must be very careful drawing conclusions because it does not always reflect reality. Two variables can be strongly correlated in many ways. For example, the number of libraries in a city is strongly related with the absolute number of crimes. However, that does not mean that libraries encourage crime. This example is very clear, and it sure is easy to find more examples of this kind, but I would like to mention the spurious correlations. These correlations ocurre when two variables with no logical connection have a strong correlation coefficient. For instance, the annual number of PhD in computer science in the United States and annual revenue of Americans arcades. …… In Tyler Vigen’s website you can find more of those correlations and if you are really interest, you can buy his book on the subject, Spurious Correlations.