With the widespread adoption of the Web, many compa- nies and organizations have established websites that pro- vide information and support online transactions (e.g., buy- ing products or viewing content). Unfortunately, users have limited attention to spare for interacting with online sites. Hence, it is of utmost importance to design sites that attract user attention and effectively guide users to the product or content items they like. Thus, we propose a novel and scal- Able experimentation approach to evaluate the effectiveness of online site designs. Our case study focuses on the effects of an authority message on visitors' browsing behavior on workshop and seminar online announcement sites. An au- Thority message emphasizes a particular prominent speaker and his/her achievements. Through dividing users into con- Trol and treatment groups and carefully tracking their online activities, we observe that the authority message influences the way users interact with page elements on the website and increases their interests in the authority speakers.