Most people who die in car crashes have eaten a hamburger less than a week before the tragic event cuts their lives short. Does this mean eating hamburgers cause traffic accidents? Nope. A connection between the two events has to be established before you can unfurl and plant the “cause and effect” flag.
That's why, when it comes to medical issues, there needs to be numerous studies pointing in the same direction. Studies with mixed results suggest there could be other causes at work besides the one we are investigating.
Consider this: Rich people may live longer because they have access to better health care. Do they live longer because they are rich? Well, sort of. That's what gives them access to the better health care.
The whole cause/effect thing gets especially confusing when things happen around the same time frame. We have a natural desire to tie them together with a big bow. Remember the saying about “trouble coming in threes”? When we begin looking for groups of three, we tend to remember those times when our hypothesis was confirmed. We think it’s true because we don’t notice or simply discount situations when life didn’t fit with our triplet theory.