Year-over-year change describes movement from an earlier count. Months of supply relates current inventory to the market's sales pace.
Cook County inventory was down 8.26% year over year in May 2026. DuPage was up 2.19%, and Lake was down 2.42%. Yet the reported months-of-supply values were close: 2.8 in Cook and 2.6 in both DuPage and Lake.
The results are possible because the measures answer different questions.
Direction is not level
Year-over-year inventory change compares the current count with the count one year earlier. It says whether inventory moved up or down and by how much relative to that earlier base.
Months of supply relates available inventory to a sales pace under the provider's method. Two counties can have the same reported supply duration even when one added inventory and the other lost it, because their starting levels and sales activity differ.
A positive change does not automatically mean loose supply
DuPage's inventory increase did not produce a higher months-of-supply figure than Cook's. Conversely, Cook's larger inventory decline did not by itself reveal how quickly an individual property would attract a contract.
Other May measures reinforce the need for separate definitions. Median days on market ranged from 42 to 46 days, while the share going off market within two weeks ranged from 52.09% to 63.97%.
Counts and rates should remain separate
The county inventory counts were 14,057 in Cook, 2,429 in DuPage, and 2,055 in Lake. Those totals differ partly because the counties and their housing stocks differ in scale. A raw count is not a competitiveness ranking. The year-over-year rate normalizes each county to its own earlier count, while months of supply introduces the provider's sales-pace denominator. Combining those measures into one unsupported score would hide the distinctions the archive preserves.
The same caution applies to market speed. Median days on market describes a middle observation, while the two-week share counts the portion crossing a threshold. Neither tells a particular property's expected time to contract.
Keep the county boundary visible
These are countywide, all-residential historical observations. They are not current September conditions and cannot be relabeled for a city, neighborhood, suburb, price band, or property type.
A buyer or seller should use them to ask for the narrower active, pending, and closed competitive set that applies to the property.
Source and method: Redfin Data Center downloads and metric definitions, all residential, 30-day, not seasonally adjusted, May 2026. Read the full county study for the exact archive rows and limits.