How do you measure what affect the CEO had? CEOs (like Jack Welch for example) get great acclaim for years according to short term benchmarks only to be proven

How do you measure what affect the CEO had? CEOs (like Jack Welch for example) get great acclaim for years according to short term benchmarks only to be proven to have made disastrous decisions for the business. Appraisal of CEOs is more witchcraft than science.