I learned while working for the Census Bureau that Illinois is the special census capital of the country. (BTW, I’m from out of state.) In fact, the Census office that manages them is in Chicago. A special census can be taken whenever a sufficient part of a jurisdiction has believed to have grown in order to capture more state turnback funds that are allocated according to decennial census counts and take into consideration population changes. Illinois is notorious for partial censuses, which only enumerate a part of a jurisdiction. These are often used to inflate counts by ignoring population losses in unenumerated areas. The fix is to allocate funds using US Census Bureau population estimates, as many other states do. This way, neutral third-party estimates can be used to allocate funds and account for (estimated) population changes. I used to work on these.