model the act of pointing, either by physically touching an object with a hand or finger, or virtually, by pointing to an object on a computer monitor using a pointing device. In other words, this means for example, that the user needs more time to click on a small button which is distant to the cursor, than he needs to click a large button near the cursor. Thereby it is generally possible to predict the speed which is needed for a selective movement to a certain target. Mathematical formulation The common metric to calculate the average time to complete the movement is the following: where: MT is the average time to complete the movement. a and b are constants that depend on the choice of input device and are usually determined empirically by regression analysis. ID is the index of difficulty. D is the distance from the starting point to the center of the target. W is the width of the target measured along the axis of motion. W can also be thought of as the allowed error tolerance in the final position, since the final point of the motion must fall within ± of the target's center. This results in the interpretation that, as mentioned before, large and close targets can be reached faster than little, distant targets. Applying Fitts' Law in user interface design As mentioned above, the size and distance of an object influence its selection. Additionally this effects the user experience. Therefore, it is important, that Fitts' Law is considered while designing user interfaces. Below some basic principles are mentioned. Interactive elements Command buttons for example should have different sizes than non-interactive elements. Larger interactive objects are easier to select with any pointing device. Edges and corners Due to the fact, that the cursor gets pinned on the edges and corners of a graphical user interface, those points can be accessed faster than other spots on the display. Pop-up menus They should support immediate selection of interactive elements in order to reduce the user's "travel time". Options for selecting Within menus like dropdown menus or top-level navigation, the distance increases the further the user goes down the list. However in pie menus, the distance to the different buttons is always the same. In addition, the target areas in pie menus are larger. Task bars To operate a task bar, the user needs a higher level of