Suppose that your friend Oscar has collected data and determined that towns with newly constructed high schools tend to have higher SAT scores than other towns
. He tells you that he has proven that new high schools cause higher SAT scores.
When you object that "correlation does not imply causation," he is ready with more data. He shows you convincing evidence that SAT scores tend to increase shortly after towns build new high schools, but that there is no tendency for new high schools to be built in towns which have recently seen large increases in SAT scores.
Is this enough evidence to prove that new high schools cause higher SAT scores, or can you think of an alternative explanation for Oscar's data?