Explain the error in the conclusion reached by a security analyst:

The Dow Jones Industrial Average peaked at 381.17 during September 3, 1929. The socalled Great Crash pushed this index down 48% by November 13. But, by April 17, 1930, the index rebounded 48% from the November bottom. In other words, anyone who bought a diversified portfolio of stocks during September 1929 would have experienced no net change in the value of his portfolio by April of 1930.