Applicability of Fitts law in expert user scenarios

Question

Fitts law states that "the time required to rapidly move to a target area is a function of the distance to the target and the size of the target.".

Simply put, if user is about to click/tap/press a button on a screen or keyboard, it is going to take him less time to do so if button is bigger and it is physically closer to his mouse pointer/his fingers.

The referenced Wikipedia article does not however mention whether there have been any tests conducted with expert users - i.e. people familiar with the system, who tend to carry out actions faster and "chain" those automatically, without too much thinking.

Let's take, for instance, a common action, requiring different keystrokes on two different platforms: opening a file. On Mac OS X, in Finder, user is needs to press ⌘-O, on Windows, in Explorer, Enter.

While there probably is a difference in how quick this operation can be carried out by an untrained user, with "Enter" key being the fastest scenario due to the obvious reasons (only one key to press, key itself is generally bigger than "O" etc), I wonder if this difference becomes insignificantly small when tested on expert user base.

Are there any studies exploring the area of Fitts law applicability to expert user base and what are the results?

Also, as a side question: why on OS X ⌘-O has been chosen for such a common action as opening a file, and Enter for rename operation, which user needs to carry out way less frequently?

Answer 1

Even if a user is an expert, Fitts' law applies. Fitts' law doesn't refer to finding/identifying the target so much as how long it takes to hit it. Even after a user has developed muscle and spatial memory for where to move the cursor, a larger target that's closer to the cursor is still going to be quicker to hit than a smaller one further away (if only because a larger one is more forgiving).

Your example of keyboard shortcuts is interesting. Firstly (as an aside) I use ⌘↓ to open a file or folder on OS X. I'm not aware of any research for whether an expert user can execute such a command more quickly than an expert user might be able to target the Enter/Return key, but there are two factors that may influence it:

1. A power user is likely to have their fingers more-or-less arranged on the home row, which (for me) puts a ⌘ key less than 3 cm away from each thumb and the O key less than 3 mm from my finger (or even directly underneath it).

2. I can target the ↓ key extremely quickly because it is in a very predictable place with few similar keys around it (even though it's further from my hand's usual position). When I target the ↓ key, I move my hand to the bottom-right corner of the keyboard and my fingers naturally sit over all four arrow keys. That makes it feel relatively quick to target, despite being the smallest key on the entire keyboard.

3. Keyboard shortcuts are also designed to be memorable (at least for English users); ⌘O is a very predictable shortcut for that operation, and easy to memorise. It follows the same pattern as ⌘N and ⌘P.

4. The standard Mac shortcut is also able to be used while editing a document. ⌘O is the shortcut I'd use while in a Pages document to open a new document (whereas the Return key is naturally going to insert a carriage return into the document instead).

I do agree, though, that there's no real reason for the Return key to edit a file name. It's actually an extremely unusual case because it doesn't even appear as a menu item (which makes the help menu behaviour very different compared to other similar operations like copy and paste). If I want to rename a file using the mouse, I have to click directly on the name of the file then wait a beat before it becomes editable. The OS X help (at least in Lion) instructs users to press Return to rename a file or folder (which is, to my mind, extremely poor discoverability):

1. Select the item you want to rename and press Return.
   Note: The items in the Finder window sidebar are aliases. To change the name of a sidebar item, locate the original and rename it.
2. Type a new name for the item.
   You can use numbers and most symbols. You can’t include a colon (:) or start the name with a period (.). Some applications may not allow you to use a slash (/) in a filename.
3. Press Return.