Edward Harrison has a great article on GDP calculation on nakedcapitalism:
Recovery does not mean recovery
My final thought on the statistics here has to do with starting from a lower base. Before the Great Depression in 1929, the U.S. had nominal GDP of $103.6 billion. By 1933, this had dropped to $56.4 billion due to deflation and a decrease in production. Over the next four year, GDP growth soared. It was 17.0% in 1934,11.1% in 1935, 14.3% in 1936 and 9.7% in 1937. That’s some serious growth, right? Well, GDP was only $91.9 billion in 1937, a full 11.3% lower than it had been 8 years earlier. In fact, it wasn’t until 1941 that we attained the nominal production output of 1929.
Read the whole thing to find out what all that means.