As 2017 came to an end, the U.S. Census announced its end-of-year U.S. state population estimates. These numbers indicate that Pennsylvania has once again passed Illinois to regain its position as the U.S. state with the fifth largest population. Both states have more than 12.8 million residents.
Census end-of-year estimates are based upon statistical analyses of population trends and are not based on actual physical counting. The U.S. Constitution requires the Census to physically count all Americans once every ten years, with the next true census set to be taken in 2020. Households will receive paper forms in the mail to fill out and send in, with census takers physically fanning out to visit households that do not return the forms.
The end-of-2017 Census announcement marked a reversal from the late 1990s, when Illinois (the 6th largest state in 1990) passed Pennsylvania (the 5th largest state in 1990, as it is again in 2018). Illinois was #5 in the 2000 and 2010 censuses. Both states have had almost stable populations for the past thirty years.