Movement fitts law

Question 1

Fitts’ Law

Answer 1

Fitts’ Law is a model of human performance

Question 2

Fitts’ Experiment – Apparatus

Answer 2

• Four distances (D):
  2, 4, 8, 16 inch
• Four widths (W):
  0.25, 0.5, 1, 2 inch
• 16 combinations

Question 3

Fitt’s Experiment – Conclusions

Answer 3

• Movement time depends on the task
• A pointing task has two properties that affect performance
• Target distance (= Amplitude of the movement)
• Target width (= Tolerance for landing on the target)
• When a target is nearer, we can reach it faster
• When a target is smaller, we have to slow down to land on it

Question 4

Index of Difficulty (ID)

Answer 4

ID = log2(D/W +1)

Question 5

Building a Fitts’ Law Model - Formulation

Answer 5

Movement time
depends on task
difficulty

The relationship
is linear
* Fitts’ Law:
𝑀𝑇 =𝑎+𝑏∗ID

Question 6

Fitts’ Law – Generalisation

Answer 6

• Distance (D): Distance of the target
• Width (W): width of the target in the
  direction of the movement
• ID: index of difficulty of the task, in bits
• b: rate at which time increases with
  task difficulty, in seconds/bit
• a is a time constant, in seconds

MT= a+b * ID = a+b * log2(D/W +1)

• D, W and ID are properties of the
  movement task
• independent
  of the device
  used for the movement
• a and b are
  device-dependent, on
  the device and body part used to
  perform the movement

Question 7

Speed-Accuracy Trade-off

Answer 7

• Fitts’ Law captures the speed-accuracy trade-off in movement
• We move faster, when we don’t have to be accurate
• We can be more accurate, when we move more slowly
• The speed-accuracy trade-off is a fundamental property of
  input in user interfaces
• When we move faster, we make more errors
• Pointing with less precision
• Typing errors

Question 8

Factor of Device

Answer 8

Speed and accuracy of user input depend on the input method.

Influencing factors include:

User’s motor movements (e.g., eyes, hands, head)

Input devices used (e.g., mouse, trackpad, joystick)

How movements are mapped to the screen (e.g., control-display gain, transfer functions)

Question 9

Example: Control-Display Gain (CD Gain)

Answer 9

CDgain = Vdisplay/ Vcontrol

CD gain > 1: less movement of the input
device needed, for faster cursor movement
on the display (coarse pointing)

High CD gain = Flick the mouse a little, and the pointer flies across the screen.

Low CD gain = Move the mouse a lot, and the pointer moves just a little — good for accuracy.

*CD gain <1: cursor moves more slowly than
the input device, for precise input
(fine positioning)

Question 10

Throughput

Answer 10

Throughput is how much data passes through a system over time.

It depends on bandwidth (speed) and signal-to-noise ratio (accuracy).

It combines speed and accuracy into one efficiency measure, given in bits per second.

Question 11

Throughput in Fitts’ Law

Answer 11

Fitts’ Law defines throughput TP as a measure of input efficiency

TP = ID/MT