Chicago media were all over the story this week that Chicago was the only major American city to lose residents in 2015. The Chicago Tribune summed it up this way:

This city has distinguished itself as the only one among the nation’s 20 largest to actually lose population in the 12-month stretch that ended June 30.

Almost 3,000 fewer people live here compared with a year earlier, according to new figures from the U.S. Census Bureau, while there’s been a decline of more than 6,000 residents across the larger metropolitan area.

Chicago’s decline is a mere 0.1 percent, which is practically flat. But cities are like corporations in that even slow growth wins more investor confidence than no growth, and losses are no good at all.

The last paragraph cited above is a good one; 3,000 people either way is not very many and this is all about perceptions.

But, there is a larger issue at stake. These population figures are estimates. Estimates. They are not exact. In other words, the Census Bureau doesn’t measure every person moving in or leaving for good. They do the best the can with the data they have to work with.

For example, on May 19 the Census released the list of the fastest growing cities in America. Here is what they say about the population figures:

To produce population estimates for cities and towns, the Census Bureau first generates county population estimates using a component of population change method, which updates the latest census population using data on births, deaths, and domestic and international migration. This yields a county-level total of the population living in households. Next, updated housing unit estimates and rates of overall occupancy are used to distribute county household population into geographic areas within the county. Then, estimates of the population living in group quarters, such as college dormitories and prisons, are added to create estimates of the total resident population.

If you want to read the methodology behind producing the 2015 city population figures, read the two page document here.

So why doesn’t the Census and the media report the margin of error? What exactly is the margin of error? For a city of Chicago’s size – just over 2.7 million – couldn’t a loss of 3,000 residents actually be a small gain in population or a loss double the size? New York’s gain of 55,000 people in 2015 seems pretty sure to be positive regardless of the margin of error. But, small declines – as published here in USA Today – seem a bit misleading:

I know the media and others want hard numbers to work with but it should be made clear that these are the best estimates we can come up with and they may not be exact. I trust the Census Bureau is doing all it can to make such projections – but they are not perfect.