Fitt's law and menus

Frank Ritter

20 Nov. 1997, revised 12 Dec. 97, 22 aug 2000, 30 aug 2000

According to a simple version of Fitt's law (taken from Card, Moran and Newell, 1983, The Psychology of Human-Computer Interaction), the time to move to a target takes about

100 ms * log2 (Dist/target + 0.5).

100 ms * log2 (2* Dist / target) has also been used according to Card et al. They are different, but taking the log of the result means they are quite similar in the end. If you look around, there are other corrections available, but I believe they all use log and dist/target.

There are also more complete and thus more accurate versions, but consider this one for now since it illustrates the basic relationships.

Distance is usually taken to be the middle of the target, and the target's size is measured in the direction of movement. Clearly, target width orthogonal to movement has some role (or else a 200 item pie menu would be fast indeed), but is not included in this simple equation.

Assume time to press and release a button is 150 ms.


Consider a pull down menu

 ---- X ----
 -----------
 |  title  |
 -----------
 |  item1  |     with X indicating where the mouse starts
 -----------
 |  item2  |
 -----------
     etc.

Compute the average time to select each item on a 9 item menu. You will have to make and use assumptions about the size of the menu.

 


Consider a square popup menu

 -----------
 |  |   |  |
 -----------
 |  | X |  |   with X indicating where mouse starts
 -----------
 |  |   |  |
 -----------


Compute the average time to select each item on a 9 item menu.

 


 

Consider a popup pie menu

    ------
  / \   /  \
 | ___x____|    mouse starts at 'x'
 |         |
  \  /  \  /
    ------
 

Compute the average time to select each item on a 9 item menu (only a 6 item menu is shown).