Useful statistics have two features. First, they are persistent, which means what happens in the present is similar to what happened in the past. If the job you do is predominantly a matter of skill, you can expect to be able to repeat your performance reliably.
Good statistics are also predictive of the goals you seek. Let’s say that we keep track of the percentage of shots that a player makes in a basketball game, and the goal of the team is to score points when playing offense. We see that, all things being equal, the higher percentage of shots a player makes, the more points he produces…This is an obvious case, of course, but not all relationships between cause and effect are as clear.