A scrollbar is the most basic function of a graphical user interface. It is usually displayed on one side of an application window when a displayed document is larger than the window. However, the scrollbar is mostly presented as a simple bar without much information, and there is still plenty of room for improvement. In this paper, we propose an overview scrollbar that displays an overview of the entire document on it and implemented four types of overview scrollbars that use different compression methods to render the overviews. We conducted a user study to investigate how people use these scrollbars and measured the performance of them. Our results suggest that overview scrollbars are more usable than is a traditional scrollbar when people search targets that are recognizable in overview.