Marketing concept that simply refuses to die.
Before a new product hits the shelves, advertising companies run it through a series of test runs to determine whether the product has national (or international) appeal as well as to determine whether the advertising needs to be tweaked to target a different sector of the population. These trials are generally limited to medium-sized towns or suburbs (about 50,000 people or so) with a diverse set of inhabitants to fully capture the nation-wide demographic.
It was discovered in the 1970's that the town of Aurora, Illinois was an ideal place to try these new marketing campaigns as the population almost exactly mirrored national statistics in terms of income, age, education level and racial make-up. So many new products were initially run in Aurora that these tests became part of standard advertising practice. Due to the town's proximity to Chicago (and, realistically, because these people were advertisers and 'Aurora Trials' didn't sound as good) these tests became known as Chicago Trials.
The problem is that the demographic of a town like Aurora isn't static - as time goes on and living conditions change those statistics shift. Nowadays Aurora isn't anything close to as perfect a test market as it used to be, yet because Chicago Trials have become a part of the lexicon (and because business directors still ask about them, regardless of their relevance; old habits die hard) they still go on, though admittedly not as much as they used to, at least not specifically IN Aurora itself.
More and more, the term Chicago Trial is being used to refer to any small scale test market, regardless of where that test market actually is.