Buying quality stocks should be one of the basic principles while investing in markets. But when one is buying mid-cap stocks this principle becomes even more important for multiple reasons and specially at a time when valuations are not cheap and small corrections can lead to strong negative reaction in stock prices. Now how does one do it? Look at the underlying business which is best understood by going through its annual report and look at a certain basic ratio in order to figure out how much return that underlying business can generate in best and worst case.