Event studies are one of the most powerful and widely used applications of the capital asset pricing model (CAPM). An event study is an attempt to determine whether a particular event in the capital market or in the life of a company has affected a company's stock market performance. The event-study methodology aims to separate company-specific events from market- and industry- specific events, and has often been used as evidence for or against market efficiency.

1. An event study aims to determine whether an event or announcement caused an abnormal movement in a company's stock price. The abnormal returns (AR) are calculated as the difference between a stock's actual return and its expected return, where the stock's expected return is typi- cally measured using the market model, which relies only on a stock's market index to estimate its expected return.

2 Using the market model can measure the correlation between an individual stock's return and its corresponding market returns. In some cases, we sum the abnormal returns to arrive at the cumulative abnormal return (CAR), which measures the total impact of an event through a particular time period, also called the event window.