Correlation

Have you noticed that the stock price of some companies is closely followed by another one, usually a rival in the same sector? The theoretical explanation is that, because these two companies are in the same type of business, they share the same challenges, require the same materials and resources, and compete for the same type of customers.

You could think of many possible pairs, but you would want to check whether a real relationship exists. One way is to have a look at the correlation of the stock returns of both stocks. A high correlation implies a relationship of some sort. It is not proof though, especially if you don't use sufficient data.