For example, if you looked for a relationship between the number of points scored by a basketball team and the number of books checked out of a library on different days, you might find a significant relationship. This doesn't mean that the score of basketball games causes people to check out library books or that checking out library books causes the basketball team to score more points. The reason why these two activities vary in a similar fashion is completely unknown and untested. It just happens that the two measurements vary in a related fashion. Maybe there was a lot of rain when the basketball scores and library books were counted.
Some experiments do show that on days with a full moon there is more abnormal behavior.
However, many of these studies have been criticized because they were not performed properly. Unfortunately, the occasional newspaper story that describes strange behaviors during a full moon only reinforces this myth. The full moon appears once every 29.53 days. Sometimes there are two full moons in one month. It IS possible to have a month without a full moon, but this does not happen very often and it can happen only in the month of February.