For the second summer in a row, writers of the Heard on the Street, The Wall Street Journal’s business and financial analysis column, are slapping “buy” or “sell” ratings on stocks. It is an exercise that is in keeping with the Heard’s long history of taking a stand on issues that matter for investors, but it also runs up against a giant caveat: Not many people are good at picking stocks, and their successes are a result of luck more than anything else.

It...