Transcript Document

RMH: Fitts’ Law
Paul Cairns
[email protected]
A law?!?!




One of the few in HCI
Predictive
Reliable
Valuable research tool!
ARMH: Fitts' Law
Today’s objectives





Fitts’ Law
Theoretical basis
Adaptations for HCI
Implications for design
Thoughts on modelling
ARMH: Fitts' Law
Overview
 Model for prediction
 Time to point
 Difficulty of target
ARMH: Fitts' Law
A demo
Interactive Fitts' Law talk
– Not quite accurate!
ARMH: Fitts' Law
Fitts’ Proposed Law
2D
T a b.log2
W




D
1/W
a, b
Log?
ARMH: Fitts' Law
Theory (or Analogy?)
 Analogy with Shannon information
S N
C B.log2
N
 Meyer’s derivation
 MacKenzie’s improvement
D W
T a b.log2
W
ARMH: Fitts' Law
Terms
 Index of difficulty
– bits
D W
ID log2
W
D
log2
W
1
 Index of Performance, 1/b
– bits per second
ARMH: Fitts' Law
Impact in HCI
 Reduce ID
– Bigger icons, more space
 Compare IP
– “Capacity” of input devices
 Put things in edges and corners
ARMH: Fitts' Law
Deconstructing Fitts
 Ecological validity
 Construct validity
ARMH: Fitts' Law
What Fitts did:
W
D
ARMH: Fitts' Law
Let’s have a go!
ARMH: Fitts' Law
What we apply it to:
ARMH: Fitts' Law
Correcting for W






W’ – actual cross-section
Smaller of W and H
Area, W x H
Sum, W + H
Stick with W
Which is best?
ARMH: Fitts' Law
Implications debunked




Edges are better
Corners are best
What about closeness of targets?
Mice are non-linear anyway!
ARMH: Fitts' Law
What remains?
 D/“W” is key
– Target size (angle)
– Stopping range (proportion)
 Non-linear (concave), monotonic
– Quite possibly log function
 IP is meaningful
 a is important
ARMH: Fitts' Law
Toolbars
 This is annoying not useful
 Edges and corners?
ARMH: Fitts' Law
Novel interactions
 Artificially increasing W
– “Sticky” buttons
– Bubbles
 Changing select
– Goal-crossing
ARMH: Fitts' Law
Novel devices
 Comparing IP
–
–
–
–
iPhone
Wii
Kinect
Eye Gaze
ARMH: Fitts' Law
Thoughts on Modelling
 Is it a good model?
– Yes, it fits the data
– No, we don’t know why!
 Could we produce a better one?
– How?
ARMH: Fitts' Law
Advanced Fitts’ Law
 Fitts’ law as a model
 Steering law
– Games
– Menu navigation
– VE/VR?
ARMH: Fitts' Law
Reading for today
 MacKenzie (1992) Fitts’ Law as a Research
and Design Tool…, HCI (7), 91-139
 MacKenzie & Buxton (1992) Extending Fitts’
Law to 2d tasks, CHI 1992, 219-226
 Interaction Design, 2nd edn
 Cockburn & Firth (2003) Improving the
acquisition of small targets. BCS HCI 03, 181196
 Accot & Zhai (1997) Beyond Fitts’ Law…
ACM CHI 97, 295-302
ARMH: Fitts' Law