Researchers have dealt with this problem by using Shannon's formulation in calculating ID, or by using a different interpretation for target width. Fitts' law often yields low or negative ID values when applied to two-dimensional target acquisition tasks. The model breaks down because of problems in interpreting target width. In addition, when the amplitude of a target is very small the ID can become very low. Positioning time in Fitts' law is the time measured from when cursor movement starts until the selection button is pressed.
Researchers often come up with different prediction equations for the same device.
Naturally, the question arises as to which equation is correct. So, incorporating Fitts' law prediction equations into design tools will remain just a thought until research can validate the measures found. A serial task could involve assembly line work. Reaching for parts and then placing them in their places as the line moves is a serial event. A designer puts together a series of screens, their buttons, menus, icons, etc.