I highlighted one recent prediction that Chicago would soon trail Houston in population. Yet, another projection has Chicago gaining people and holding off Houston for longer. Which is right?

Data released by the Illinois Department of Health in February show that the population for Chicago, about 2.7 million in 2010, could decrease by 3 percent to 2.5 million by 2025. Meanwhile, Houston’s population could reach 2.54 million to 2.7 million in 2025, according to the Reuters report. But a recent population estimate by the Census Bureau shows an increase in population, rather than a decrease.

Census estimates released in June show that the population of Chicago increased by 1 percent from 2010 to 2014. So why is one projection showing a decrease, but another an increase?

Both data sets are based on estimates and assumptions, says Rob Paral, a Chicago-based demographer. Unlike the 2000 or 2010 census, where all residents answer a questionnaire, any interim projections or estimates must use sampling or a formula based on past population statistics to calculate population…

“Trend data do not support any increase in the projections for Chicago in the next 10 years,” said Bill Dart, the deputy director of policy, planning and statistics at the health department. Dart explained that the estimates from the census use a different formula than the health department. And factors such as births, deaths, migration, economic boons or natural disasters can disrupt projections.

Two groups dealing with population data that come to opposite conclusions. Two ways we might approach this:

1. The differences are due to slightly different data, whether in the variables used or the projection models. We could have a debate about which model or variables are better for predicting population. Have these same kind of variables and models proven themselves in other cities? (Alternately, are there factors that both models leave out?)
2. Perhaps the two predictions aren’t that different: one is suggesting a slight decline and one predicts a slight increase. Could both predictions be within the margin of error? We might be really worried if one saw a huge drop-off coming and the other disagreed but both projections here are not too different from no change at all. Sure, the media might be able to say the predictions disagree but statistically there is not much difference.

The answer will come in time. Still, projections like these still carry weight as they provide grist for the media, things for politicians to grab onto, and may just influence the actions of some (is Chicago or Houston a city on the rise?).