For years WGN-AM has held down its status as the ratings king of Chicago radio while urban contemporary WGCI-AM was the perennial runner-up. That was when the Arbitron ratings service used listeners' diaries to determine who was tuned in to what.
Now Arbitron has introduced "eavesdropping devices known as portable people meters" that present such a startlingly different picture that much of what we thought we once knew about local radio listeners may turn out to be wrong.
Under the new system, WGN is still on top, but WGCI drops to 14th while WDRV-FM -- known as The Drive -- jumps into second.
You can imagine how much this shifts the landscape; all those advertising dollars - and salaries - all these years based on ratings that may have been way off.
On the other hand, the new system has its critics too.
"African-American and Hispanic station owners have argued that the new system will dramatically undercount listeners, particularly minority youth, because of low sample sizes, faulty recruiting and flawed methodology," Kristal Brent Zook writes in "Death of Black Radio" at TheRoot.com.
That concern led Barack Obama - and Richard Durbin - to call for a delay in implementing the people meters because "the implications of poor implementation of a new PPM system for the public interest are too serious to ignore."
So it may be too soon to pop the Champagne at The Drive and other stations who suddenly find themselves with more listeners than they thought, but it might not be long before a truer ratings picture of Chicago radio emerges.