Who shrank Germany?

News headlines are abuzz with the news of Germany's first census count since reunification. Unfortunately, they seem to have run with the wrong message:

"Census reveals German population lower than thought" BBC Europe
"Germany Counts Heads and Finds 1.5 Million Fewer Residents Than It Expected" New York Times
"German 2011 Census Shows Population Smaller than Expected" Wall Street Journal
"Census shrinks German population by 1.5 million" The Local: Germany's News in English

First, a census cannot "shrink" a population...

Second, an estimate is an estimate. There is an understood margin of error.

But most importantly, yes indeed, German demographers slightly over-estimated the nation's population. Using data from the last census in West Germany (in 1987), the last census in East Germany (in 1981), birth records, death records, and resident registration records, estimated the nation's total population to be 81.7 million. Census counts show that the nation's population is closer to 80.2 million.

Being off by 1.5 million people sounds like a huge mistake. But in context, that's 1.9 percent estimation error over the twenty five years since the last census was conducted, or less than one tenth of one percent error per year without a census. That's a surprisingly narrow margin of error when you're counting something as changeable as resident population size with out-of-date data.

Clearly the headline-writers have forgotten real mathematical "oops" moments, such as:

• Malthus' theory that food production tends to increase arithmetically while population tends to increase naturally at a geometric rate, and hence, humanity was likely doomed;
• Paul Ehrlich's Population Bomb predictions that there would be mass starvation and declining life expectancy, worldwide, beginning in the 1970s and 1980s;
• Dewey Defeats Truman;
• the Arizona alternative fuel vehicle rebate debacle in 2000, estimated to cost $10 million that wound up costing more than $200 million, and eventually needed to be shut down;
• the estimated cost of Boston's "big dig" project, and
• the estimated risk of the Titanic hitting an iceberg.

(I could go on, but I suspect you get the point...)

With less than 2 percent error over 25 years, German demographers should be praised for their skill, not lambasted for a slight over-estimate.*

Thankfully, other news sources zeroed in on the real story: using demographic data to better understand the fabric of a nation.


*The over-estimate occurred because while immigrants to Germany must register with resident registration services upon arrival, and are legally supposed to notify the state when they depart, not all out-migrants bother telling the country that they're headed out. Just think of the last time you moved. Did you immediately let the DMV know that your address changed?