Week 3 L2 - Input models

Question 1

Goal crossing

Answer 1

• Selection effort is achieved by crossing a target instead of having to land on it.
• Easier than landing on a target (pointing) as you don’t have to consider overshooting the target.

Question 2

Fitts’ law formula for goal crossing

Answer 2

MT = a + b * log2(D/W + 1)
Formula stays the same but width represents the vertical width of the goal instead of the width of the target

Question 3

When is crossing-based selection used

Answer 3

• When objects are thin and would be hard to land on
• When you want to select multiple objects
• When using an input device with no button to click
• When using a pen/stylus as an input - don’t need to lift pen to tap
• When movement is jittery or tracking noisy

Question 4

Edge/corner access points

Answer 4

Edges and corners are the fastest possible access points as you do not have to worry about overshooting.
Acts more like crossing tasks than selection tasks because you cannot overshoot.

Question 5

Steering law

Answer 5

Models the movement time (MT) to traverse through a 2D tunnel of length (D) and width (W).

Question 6

Steering law formula

Answer 6

• MT = a + b * ID
• MT = a + b * D/W
• MT: movement time
• a and b: constants dependent on the pointing system/tunnel
• D: distance, length of tunnel
• W: width of the tunnel

Question 7

Keystroke-level model (KLM)

Answer 7

A model for determining how long it will take an expert user to complete a task without errors.
Uses basic input operators with time estimates for their execution.

Question 8

Types of operator

Answer 8

• Physical motor operator: pressing a key, pointing with the mouse, draw a line, move hand from one device to another
• Mental operator: mental prep of a motor action, time to ake a decision, time to look for something

Question 9

System response

Answer 9

Time the user must wait for a response from the system

Question 10

Operators

Answer 10

• K: keystroke/button press, motor operator, multiple associated times e.g., 0.12s (90wpm - good typist), 0.28s (40wpm - average non skilled typist), 1.2s (bad typist unfamiliar with keyboard).
• H: homing, moving hand across keyboard or to another device, motor operation, 0.4s
• B/BB: pressing or releasing a button (B) or clicking a button (BB), motor operation, B - 0.1s, BB - 0.2s
• P: pointing with a mouse to a target, motor operation, 1.1s
• D: drawing nd straight line segments of length ld, motor operation, 0.9snd + 0.16ld
• M: mental acts, mental operation, 1.35s
• R(t): system response time when the user cannot act, system response, time dependent on the system.

Question 11

Advantages of KLM

Answer 11

• simple model means it is easy to apply
• can be done on just sketches of an interface
• surprisingly accurate
• appropriate for routine tasks
• especially useful for comparing alternative UI designs
• useful for testing designs for frequently and repetitively used interfaces

Question 12

Limitations of KLM

Answer 12

• Assumes user is expert and that the task is well-rehearsed.
• Assumes no errors are made.
• Not appropriate for tasks that involve exploration or reasoning.