The executives who appeared on the Harvard Business Review's 2019 list of best-performing CEOs in the world showed remarkable longevity in their roles.

The average honoree has been CEO for 15 years, more than double the S&P 500 average in 2017 of 7.2 years. All these people have created tremendous value during their careers — but like most other CEOs, many have experienced short-term ebbs and flows in performance.

The 100 best CEOs in the world for 2019

For boards, that can create a quandary: How to tell when a CEO is suffering from a negative blip versus