Help me out here.

G.D.P. is important, right? I’ve been an economics reporter for only 10 months, but I think I’ve learned that much. G.D.P. — gross domestic product — is a cornerstone economic statistic used to measure a country’s economic output. Its origins are often traced back to the work of the economist Simon Kuznets, who developed a series of national income accounts for the United States government during the Depression of the 1930s. G.D.P. itself, which Foreign Policy Magazine once called “a window into an economy’s soul,” didn’t emerge until decades later.

But the events of the last couple of months have left me wondering how much more art than science G.D.P. really is.

First, in April, Nigeria changed the base year it used to calculate its output and added nearly 90 percent to its G.D.P. Overnight, Nigeria became Africa’s largest economy.

Then Italy, Britain and Spain said they were going to start factoring prostitution and sales of illegal drugs into their G.D.P. calculations, responding to new European Union requirements. Only Britain has broken out specific numbers, saying illegal activities would have added about $17 billion, or 0.7 percent, to its G.D.P. in 2009, according to the Office of National Statistics.