The S&P 500 P/E multiple of 18.66 at the end of 2010 was higher than its historical

Question:

The S&P 500 P/E multiple of 18.66 at the end of 2010 was higher than its historical average of approximately 15. Some financial commentators argued that this meant that the firms in the S&P 500 were, on average, overvalued at the end of 2010. Based on your analysis in Problem 18.29 and the concepts covered in this book, do you think that these commentators are right or wrong? Why or why not?