One of the standard references on the topic is an article by Bruce Tognazzini, the founder of the Apple Human Interface Group and a renowned usability expert about using the keyboard vs. using the mouse: "Keyboard vs. The Mouse, pt 1" (Originally published in the AppleDirect, August, 1989; Republished as Chapter 6, in Tog on Interface). Among the long discourse:
We’ve done a cool $50 million of R & D on the Apple Human Interface. We discovered, among other things, two pertinent facts:
Test subjects consistently report that keyboarding is faster than mousing.
The stopwatch consistently proves mousing is faster than keyboarding.
However, having provided one reputable reference, I will trump that with the resident $deity. In a Jeff Atwood's 2008 Coding Horror post titled 'Revisiting "Keyboard vs. The Mouse, pt 1"', he takes issue with the naive interpretation of Tog's article linked above (namely, only using the quote that I myself shown out of context), and points to another very relevant snippet from the same article:
And, in fact, I find myself on the opposite side in at least one instance, namely editing. By using Command X, C, and V, the user can select with one hand and act with the other. Two-handed input. Two-handed input can result in solid productivity gains (Buxton 1986).
Atwood concludes, wisely:
I don't think anyone would argue that learning keyboard shortcuts is faster than using the mouse to navigate and learn a program. Clearly it isn't -- it's quite painful, as anyone who has ever been stranded at a Unix command prompt can probably tell you.
However, as Tog himself notes, when the keyboard shortcut is already memorized and well understood, it's a clear productivity win.
As an example, Jeff provides this obvious one:
Let's assume that we're typing some text into a document of some kind, and we wish to save the document we're working on.... If it seems ridiculous that the mouse method:
1. Take your right hand off the keyboard
2. Place your right hand on the mouse
3. Mouse over to the File menu
4. Click File
5. Click Save
6. Place your right hand back on the keyboard
Could be measurably faster than the keyboard method:
1. Use your left hand to press Control+S
I assure you that you are not alone.
The reason why this is true is alluded to by Jeff, but to elaborate on it a bit using the basic User Interface Design 101 concepts:
- It is very easy to start moving the mouse
- it is very hard to stop moving it.
Therefore, clicking on a menu item that is in the corner (best) or at the very top edge of the screen (worse but OK) is a fairly easy UI task for a human hand-eye coordination.
Clicking on a small element of the sub-menu 3 levels deep in menu tree, on the other hand, is much harder.
Therefore, as noted in my comment, the efficiency answer depends to a large part on the specific task to be done and a specific interface (or rather its design from mouse and keyboard work perspective).