Resumen
Public utilities? earnings are inherently linked to decisions of the regulatory body. When regulators make decisions that are unexpected or that cause additional risk to shareholders one expects that such decisions will have an effect on the price of the utility?s stock. In 2010 the Illinois Commerce Commission, the regulatory body that sets electric utility rates in Illinois, made a dramatic change in the manner in which it applied an accounting rule, the pro forma adjustment rule, causing the rates for Ameren Illinois Utilities to be substantially less than otherwise would be the case. If this decision provided information to investors, we expect the Ameren stock price to react to the decision. Using a single-firm, single-event methodology we find limited evidence that this one event had an effect on daily abnormal returns to the Ameren Corporation?s stock. We also discuss possible reasons for these results.