HUMAN-COMPUTER INTERACTION
SECOND EDITION
HUMAN-COMPUTER INTERACTION
SECOND EDITION
P -- Pointing, moving the mouse (or similar device) at a target.
H -- Homing, switching the hand between mouse and keyboard.
D -- Drawing lines using the mouse.
The execution of a task will involve interleaved occurrences of the various operators. For instance, imagine we are using a mouse-based editor. If we notice a single character error we will point at the error, delete the character and retype it, and then return to our previous typing point. This is decomposed as follows:
1 move hand to mouse H[mouse]
2 position mouse after bad character PB[LEFT]
3 return to keyboard H[keyboard]
4 delete character MK[DELETE]
5 type correction K[char]
6 reposition insertion point H[mouse]MPB[LEFT] -->
Notice that some operators have descriptions added to them, representing which device the hand homes to (for example, [mouse]) and what keys are hit (for example, LEFT -- the left mouse button).
The times for the other operators are obtained from empirical data. The keying time obviously depends on the typing skill of the user and different times are thus used for different users. Pressing a mouse button is usually quicker than typing (especially for two-finger typists), and a more accurate time prediction can be made by separating out the button presses B from the rest of the keystrokes K. The pointing
As an example, we compare the two methods for iconizing a window given in Section 6.7.1. One used the 'L7' function key, and the other the 'CLOSE' option from the window's pop-up menu. The latter is obtained by moving to the window's title bar, depressing the left mouse button, dragging the mouse down the pop-up menu to the 'CLOSE' option, and then releasing the mouse button. We assume that the user's hand is on the mouse to begin with, and hence only the L7-METHOD will require a homing operator. The operators for the two methods are as follows:
L7-METHOD H[to keyboard] MK[L7 function key]
CLOSE-METHOD P[to menu bar] B[LEFT down] MP[to option] B[LEFT up]
The total times are thus
L7-METHOD = 0.4 + 1.35 + 0.28
= 2.03 seconds
CLOSE-METHOD = 1.1 + 0.1 + 1.35 + 1.1 + 0.1
= 3.75 seconds -->
The first calculation is quite straightforward, but the second needs a little unpacking. The button presses are separate down and then up actions and thus each is only timed at 0.1 of a second, rather than 0.2 for a click, or 0.28 for typing. We have also used the simplified average 1.1 second time for the pointing task. From these predictions, we can see that the L7-METHOD is far faster. In Section 6.7.1, Sam's selection rule was to use the L7-METHOD when playing blocks. To do so, he can go on playing the game using the mouse in his right hand whilst moving his left hand over the key. Thus the real time for Sam, from when he takes his attention from the game to when the command is given, is less, 2.03 seconds minus the homing time, that is 1.63 seconds. Given the method is so fast, why does Sam not use it all the time?
Perhaps the average estimates for pointing times have biased our estimate. We can be a little more precise about the CLOSE-METHOD timing
P[to menu bar] = 0.1 log2 (10.5) = 0.339
P[to option] = 0.1 log2 (4.5) = 0.217 -->
Do a keystroke level analysis for opening up an application in a visual desktop interface using a mouse as the pointing device, comparing at least two different methods for performing the task. Repeat the exercise using a trackball. Consider how the analysis would differ for various positions of the trackball relative to the keyboard and for other pointing devices.
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