The 33⅓% rule, and the counties it does not apply to
Illinois assesses at one third of fair cash value — except in Cook County, and except for farmland.
Illinois sets the general assessment level at 33⅓% of fair cash value. Every county except Cook applies that single ratio to residential, commercial and industrial property alike.
Cook County is different
Cook classifies property and assesses classes at different percentages, with residential lower than commercial. If you are in Cook, arithmetic borrowed from a downstate guide will be wrong, and it will be wrong in the direction that makes your assessment look fine when it is not.
Farmland is different again
Farmland is not assessed on sale price at all. It is assessed on agricultural economic value, derived from soil productivity indexes and expected income. A tillable acre and an identical-looking acre of pasture can carry very different assessments, and a sale down the road is largely irrelevant to either. This is why land assessed as farm sits at a tiny fraction per square foot of the residential lot beside it — not an error, a different statute.
What to do with the ratio
Use it to translate. Your assessed value ÷ 0.3333 is the county's implied opinion of your property's sale price. That is the number to test against reality — and if it is defensible, your case is about equal treatment rather than value.