Fortune magazine (March 1997) reported the total returns to investors for the 10 years prior to 1996 and also for 1996 for 431 companies. The total returns for 9 of the companies and the S&P 500 are listed below. Find a 95% confidence interval for the mean change in percent return to investors.

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Total Return to Investors Company 1986–96 1996 29.8% 27.9% 22.1% 44.5% 22.2% 43.8% 21.7% 21.9% Berkshire Hathaway 28.3% 11.8% 43.3% 25.4% 24.0% 88.3% 18.1% 131.2% 34.0% 32.1% 6.2% 20.3% Coca-Cola Mirage Resorts Merck Microsoft Johnson & Johnson Intel Pfizer Procter & Gamble S&P 500