WASHINGTON — Are United States chief executive officers overpaid?Graef Crystal, a professor at the University of California, Berkeley, tried to answer this question by comparing CEO pay at 1,000 US corporations with the return those companies gave shareholders over the short, medium, and long terms. According to his "performance IQ," Time Warner Inc. overpaid its CEOs, Steven Ross and N. J. Nicholas, by $97 million last year. They earned $99.6 million, while "competitive compensation" for a company of that size and performance would have been $2.5 million. In this extreme example, most of the CEO earnings came in the form of unusually large stock options rather than salary, the study notes. On the other hand, some CEOs are underpaid. David Glass, CEO of Wal Mart Stores, earned $980,000 last year, while his competitive compensation would be $10.9 million, according to the study, done for the United Shareholders association. "About 30 percent of the differences in CEO pay can be traced to differences in company size. But scandalously, no more than 4 percent can be accounted for by differences in company performance," Professor Crystal says.