Investigations on applicability of Fitts´ law index of difficulty to large/small screen devices

It is well-known as Fitts´ law that the time for a user to point a target on a GUI screen can be modeled as a linear function of ldquoindex of difficulty (ID)rdquo. The authors investigate whether the ID formulation is appropriate independently of device screen sizes. Result of our experiment revealed that the ID formulation may not consistently capture actual difficulty: users´ pointing performances were not consistent. The term A/W may not be appropriate because the term causes the observed inconsistency.