Fitts’ Law Index of Difficulty Evaluated and Extended for Screen Size Variations
Hidehiko Okada, Takayuki Akib · InTech eBooks · 2010
Index of difficulty formulation in Fitts' law was evaluated from the viewpoint of consistency in widget size&distance design variations. It was found that ID in Eq. (2) may not appropriately capture actual difficulty: user performances on the same device were not consistent among target designs (A, W) and (nA, nW). Based on this finding, two multiple regression models were evaluated. These models were t=F(A,W) (c.f., t=F(A/W) in the Fitts' model) which predicted the time t to point a target with the distance A and the size W. The models are found to be able to appropriately represent that W affected the index of difficulty more than A in the case of the small screen touch UI device. The models however did not work so well in the case of the large screen device, which remained to be investigated in a future research. The authors next tried to improve the Fitts' law ID formulation. Our idea was to raise A or W depending on the screen size. The modified model was found to fit well to the users' pointing data, which supports our idea.