HCI Fitts' Law

Question 1

Give the formula for Fitts’s law

Answer 1

MT = a + b log 2 (D/W +1)

Question 2

Give the formula for Index of Difficulty

Answer 2

ID = log 2 (D/W +1)

bits

Question 3

Give the formula for Index of Performance

Answer 3

IP = ID/MT
(bits/s)
AKA Throughput or Bandwidth

Question 4

Movement Time (MT) increases linearly with Index
of Difficulty (ID) means that (2)

Answer 4

Slower when targets are smaller or further away

Moving fast make you less accurate

Question 5

Why is ID and IP measured in bits

Answer 5

to reflect the information content necessary to move to the target

Question 6

What measure allows comparison of the targeting effectiveness of different devices?

Answer 6

IP

Question 7

How do humans target? Name the 2 components

Answer 7

Rapid ballistic movement towards target

Slower corrective movements

Question 8

How can we use Fitts’ Law and knowledge of

human movement to improve interaction? (3)

Answer 8

Increase W – make targets bigger
Reduce D – bring targets closer
Support the ballistic and corrective phases of targeting

Question 9

Describe Grossman and Balakrishnan’s Bubble cursor.

Answer 9

• Cursor captures multiple targets
• Bubble cursor size adapted to capture only 1 target
• Bubble cursor morphed to encompass target

Question 10

Define the Steering Law

Answer 10

T =a+b A/W

T: average time to navigate the path
A: length of path
W: width of path

Question 11

Microsoft Toolbars offer the user the option of displaying a label below each tool. Name at least one reason why labeled tools can be accessed faster. (Assume, for this, that the user knows the tool and does not need the label just simply to identify the tool.)

Answer 11

Here are two answers. You may have more.

The label becomes part of the target. The target is therefore bigger. Bigger targets, all else being equal, can always be acccessed faster. Fitts’ Law.
When labels are not used, the tool icons crowd together.

Question 12

You have a palette of tools in a graphics application that consists of a matrix of 16x16-pixel icons laid out as a 2x8 array that lies along the left-hand edge of the screen. Without moving the array from the left-hand side of the screen or changing the size of the icons, what steps can you take to decrease the time necessary to access the average tool?

Answer 12

Two separate steps may be necessary to average tool access time. Both are important.

Change the array to 1X16, so all the tools lie along the edge of the screen.

Ensure that the user can click on the very first row of pixels along the edge of the screen to select a tool. There should be no buffer zone.
This second step is vital, and is so often ignored.

Remember that Fitts’ Law states that access time is a function of distance and target size. If the target size is larger, then the time is reduced. It is reduced for a simple reason: the user need not slow down when approaching the target for fear of overshooting.

Now consider the screen edge. How deep is the target? If it were really only the one pixel it appears, it would be very hard to hit. However, the screen edge is, for all practical purposes, infinitely deep. It doesn’t matter how fast that mouse is going when it hits the screen edge, that pointer absolutely will not overshoot. Having to hit a pixel two pixels in from the screen edge takes much longer than hitting the edge itself. Use that edge. It is your friend.

Question 13

A right-handed user is known to be within 10 pixels of the exact center of a large, 1600 X 1200 screen. You will place a single-pixel target on the screen that the user must stop upon and point to exactly. List the five pixel locations on the screen that the user can access fastest. For extra credit, list them in order from fastest to slowest access.

Answer 13

No, this is not a trick question. And the first part should be immediately answerable by any interaction designer. The extra credit question is not quite as simple. But first, the locations of the five “magic pixels”:

The prime pixel is located at the current location of the mouse pointer. Popup menus make use of this pixel, showing up relative to the mouse pointer, no matter where the user may have moved it. This pixel requires zero travel and is, in effect, an infinitely large target—you just can’t miss it.

The other four pixels are located, on average, as far away from the mouse pointer as you can get. Their distance, however, is more than made up for by their target size, which is infinite in two dimensions. These magic pixels are the four corners of the screen. Throw the mouse in any direction you desire and the odds are overwhelming that if you threw it with enough velocity, it will end up in one of those four corners. This presupposes a properly designed acceleration function for the mouse.

The key to the extra credit question is in the user’s right-handedness. A right-handed user can access, in order of increasing difficulty, and starting with the point already mentioned:

• The pixel immediately at the current cursor location: Click the mouse and you’re done.
• The bottom-right corner.
• The top-left corner.
• The top-right corner.
• The bottom-left corner.

If you hold the mouse in your right hand and move the mouse, using just your wrist and hand, in the four different directions, you will see how the mechanics of your arm leads to this. The answers for a left handed person are, of course, reversed.

These differences are relatively small compared to the power of the “magic pixels.” All four corners should be used and used well.

Question 14

Microsoft offers a Taskbar which can be oriented along the top, side or bottom of the screen, enabling users to get to hidden windows and applications. This Taskbar may either be hidden or constantly displayed. Describe at least two reasons why the method of triggering an auto-hidden Microsoft Taskbar is grossly inefficient.

Answer 14

• Screen edges are prime real estate. You don’t waste an entire edge that could be housing a couple of dozen different fast-access icons just for one object, the Taskbar.
• The auto-hidden Taskbar is entirely too easy to display by accident. Users are constantly triggering it when trying to access something that is close to, but not at, the edge.
• The Taskbar would not have any of these problems, yet be even quicker to get to if it were located at any one of four corners of the display. Throw the mouse up and to the left, for example, and you’ll have a taskbar displayed. Fast access without the false triggering.

Question 15

Explain why a Macintosh pull-down menu can be accessed at least five times faster than a typical Windows pull-down menu. For extra credit, suggest at least two reasons why Microsoft made such an apparently stupid decision.

Answer 15

Microsoft, Sun, and others have made the decision to mount the menu bar on the window, rather than at the top of the display, as Apple did. They made this decision for at least two reasons:

1. Apple claimed copyright and patent rights on the Apple menu bar
2. Everyone else assumed that moving the menu bar closer to the user, by putting it at the top of the window, would speed things up.

The Apple menu bar is a lot faster than menu bars in windows. Why? Because, since the menu bar lies on a screen edge, it has an infinite height. As a result, Mac users can just throw their mice toward the top of the screen with the assurance that it will never penetrate and disappear.

Question 16

What is the bottleneck in hierarchical menus and what techniques could make that bottleneck less of a problem?

Answer 16

The bottleneck is the passage between the first-level menu and the second-level menu. Users first slide the mouse pointer down to the category menu item. Then, they must carefully slide the mouse directly across (horizontally) in order to move the pointer into the secondary menu.

Moving our hand describes an arc, rather than a straight line. Demanding that pivoted people move a mouse pointer along in a straight line horizontally is just wrong. We are naturally going to slip downward even as we try to slide sideways. When we are not allowed to slip downward, the menu we’re after is going to slam shut just before we get there.

Question 17

Name at least one advantage circular popup menus have over standard, linear popup menus.

Answer 17

With the options displayed around you in a circle, you need only move a pixel or two to enter the “slice of pie” you want. Less travel, good target size. Good design.

Question 18

What can you do to linear popup menus to better balance access time for all items?

Answer 18

You can “Fittsize” them by making those items farther away from the mouse pointer larger. They need not literally be larger, since the user is not having any trouble seeing the farther items. Instead, the mapping of mouse-to-screen could be such that, as the user pulls further down the menu, more movement of the mouse is necessary to get a corresponding movement of the pointer. In effect, you are decoupling the behavioral map of the screen from the visual map.

Other decoupling tricks include setting up local gravity, so that once a mouse pointer gets near the target, it is drawn to the target. Barriers can be erected, so that once the mouse enters an object, it is difficult for it to pass through to the other side. This can be frustrating. Having a pressure-sensitive mouse that could “push through” if pushed down upon would enable the user either to be caught by the object or to flee to the territory beyond.

Question 19

The industrial designers let loose on the Mac have screwed up most of the keyboards by cutting their function keys in half so the total depth of the keyboard was reduced by half a key. Why was this incredibly stupid?

Answer 19

The farther away the target is, the larger it must be to retain access speed. Not only did the industrial designers reduce the total size of the target, they reduced it in the very dimension that was most critical. Stupid, stupid, stupid. What they should have done was to curve the keyboard sharply upward toward the back end, so that merely lifting the finger a few degrees would access the numeric and function keys, aiding both precision and speed.