The equalization factor, and why your assessment changed without anyone visiting
The state multiplier adjusts a whole county at once. It is not a reassessment, and it is not appealable.
Illinois requires assessments to sit at 33⅓% of fair cash value. Counties drift from that. The Illinois Department of Revenue measures the drift by comparing assessments to actual sales, then issues an equalization factor — a multiplier applied to every non-farm assessment in the county.
What it does to you
If your county's multiplier is 1.05, every assessment rises 5% before the tax rate is applied. Nobody looked at your house. Nothing about it changed. This is the most common answer to "why did my assessment go up when nothing happened?"
Why you cannot appeal it
The multiplier is county-wide by design. It applies equally to you and to every comparable property, so it cannot make you unequally assessed — which is the ground an appeal argues. Appealing the multiplier is not a thing an owner can do at a board of review.
What is still worth appealing
Your own assessment underneath it. If your pre-equalized assessment is out of line with comparable properties, the multiplier magnifies that error rather than causing it. Fix the base and the multiplier applies to a correct number instead of a wrong one.
Cook County's multiplier has historically run near 3.0 because of its lower statutory ratios. A large multiplier there is normal and is not evidence of anything.