The law of averages is a rule of probability. It says that, in the long term, results will average out to their expected value, but in the short term, no one knows what will happen.
For example, casinos set up all their games so that the chances of the house winning are slightly in their favor. That means that in the long term, as long as people keep playing, the casinos are going to come out ahead, on average.
— Statistics for Dummies by Deborah Rumsey