Correlation is not the same as causation. But it can be fun to play with correlation, especially when causation starts to grow cloudy. As the deadline nears for entry in the Union-Tribune’s 17th annual Precipitation Prediction Contest, here’s some correlation to chew on, if you haven’t already entered: Since 1850, San Diego has recorded at least 0.5 of an inch of rain in October 44 times, including this year (0.57). That’s a little more than one fourth of the years. Of those 44 years with a wet October, the entire season ended up wet 26 times, or about 59 percent of the time.