How Do You Compare Berkshire's Book Value Per Share With the S&P Average?

Carol Costello: Buffett and Munger, I think of you as running a rational company. But when I look at your annual comparison of Berkshire's book value per share versus the S&P average, I don't see any rationality in that at all. What is the logic of comparing a stock market index against the rise in an operating company's book value? And an operating company is predominantly what Berkshire is these days. So why do you annually make this irrational comparison?