Psychology of HCI

1
Psychology of HCI

• Perception
• Motor skills (interaction)
• Power law of practice
• Memory
• Ref Handbook of Human Computer Interaction (in
  library)

2
Psychology and HCI

• Abilities and limitations of humans is a very
  important influence on HCI
• perception
• motor skills
• memory
• reasoning (too hard)
• Cognitive Psychology

3
Be Cautious

• Take all figures, formulas as rough
  approximations
• Im ignoring lots of complexities!
• Individuals vary!
• But remember general messages

4
Perception Text

• Mixed-case is read 13 faster than all caps
• Proportional fonts (Arial, Times) read 6 faster
  than non-pro (Courier)
• Diffs between Serif (Times) and Sans-Serif
  (Arial) smaller, more controversial
• Avoid novelty fonts, theyre slow to read
• 8-12 pt read 5 faster than 6pt, 14pt
• Summary
• Follow defaults in Word, etc
• They make sense, theyre not arbitrary

5
Compare

• Mary had a little lamb
• MARY HAD A LITTLE LAMB
• Mary had a little lamb
• Mary had a little lamb
• Mary had a little lamb

6
Colour

• People can only recognise 20-50 colours (millions
  can be distinguished). Recognition is slower with
  more colours
• Use just a few colours for colour coding
• 8 of European adults are colour-blind
• provide alternatives to colour
• Colours have associated meanings (eg, red means
  danger, hot, stop)
• respect these, dont use red for OK

7
Colour example
Wendys changes Bobs changes This is some text
that has been changed by two people, Bob and
Wendy. Can you tell who made which change?
8
Perception Screen density

• The time taken to find information on a screen
  increases with the amount of information on the
  screen
• if we know what user wants, dont add extraneous
  information
• standard vs scientific mode on calculator
• Observation Most screens have about 25
  information density

9
Freds address v1

• Name Fred Smith
• Age 26
• Occupation programmer
• Address 10 Don St, Aberdeen
• Employer Jazzy Software PLC
• Marital status single

10
Freds address v2

• Name Fred Smith
• Address 10 Don St, Aberdeen

11
General Lessons

• Follow standard choices
• Arial or Times, mixed case, 10/11/12 pt
• Dont go against cultural conventions
• Use green for OK, not red
• Dont add extra information (functionality) just
  because you can
• will slow the user down

12
Motor skills Typing

• Typing speed improves with power law of practice
• Tn T1n-a
• T1 time to first perform task
• Tn time to perform task on nth iteration
• a constant (roughly 1/3)
• General law that applies to many practised
  actions.

13
Power law typing speed

• Speed cube-root(pages-types) 0.2
• young child (1 page typed) 0.2 char/sec?
• Uni student (1000 pages typed) 2 char/s?
• Expert (100K pages typed) 10 char/s?
• Some people type 100x faster than others
• Typing speed increases rapidly at first, then
  more slowly

14
Use keyboard?

• CTRL-S vs mouse Save button
• assume takes .5s to press Save
• child should use mouse student, expert should
  type
• CTRL-O mydoc vs press Open, sel mydoc
• assume press, sel takes 1.5s
• child, student should use mouse expert should
  type
• Choice depends on skill with keyboard!
• In fact mouse skill also improves with practice

15
Dvorak keyboard
16
Keyboards

• Dvorak typing may be 5 faster than standard
  QWERTY, with equal practice
• D speed cube-root(pages-types) 0.21
• Q speed cube-root(pages-types) 0.20

17
Switch?

• Expert 100KP Q typist (10c/s) becomes
• immed (0.1P) D typist - 0.105 c/s
• after 3 month (1KP) D typist - 2.1 c/s
• after 20 yr (100KP) D typist - 10.5 c/s
• would have reached 11.7 c/s if had stayed with Q
  keyboard and had 200KP experience

18
Keyboards

• Experienced QWERTY typist shouldnt switch to
  Dvorak
• small in-principle gain isnt worth huge loss of
  expertise, practice
• common theme in HCI!
• Could train new typists in Dvorak
• but people dislike because most typing jobs use
  QWERTY

19
Common Analysis in HCI

• X is slightly better than Y in principle
• But if Y is established, it isnt worthwhile
  switching to X
• Huge loss caused by the loss of expertise with Y
  outweighs the small gain resulting from use of X

20
Mouse Fitts Law

• Time to point (finger, mouse, whatever) depends
  on log(D/W)
• D distance pointer must move
• W size of target
• Lesson
• put things moused together close together
• use large targets (active regions)

21
Target size
Small target slow to point to
Large target quick to point to
22
Mouse speed

• Skilled mouse user
• 0.2 sec for nearby/large target (click button in
  dialogue box)
• 1 sec for far-away/small targets (select
  character in document)

23
Mouse vs keyboard

• Depends on whats been practiced
• skilled typist with little mouse experience will
  prefer keyboard
• skilled mouse user with little typing will prefer
  mouse
• Mouse integrates with screen (vision)
• good provides help, guidance to user
• bad requires eye to look at screen

24
Memory

• Short-term (working) memory holds information
  actively being used (thought about, reasoned
  with) by human
• Long-term memory permanent storage
• very large, retained indefinitely (although
  retrieval can become difficult)

25
Short-term Memory

• Small 7 /- 2 items
• items are cognitive primitives letters, English
  words, shapes, colours,
• new items formed, eg passwords
• Info decays in seconds
• Items displaced by new items coming in
• Fast lt50ms access time

26
Short-term memory

• Users perform best if all info they need is in
  STM
• group/chunk information
• 1 English word same load as 1 letter!
• Avoid irrelevant information, distractions
• displaces important info from STM

27
Example

• Microsoft Office Assistant (paper clip)
• animated character,makes suggestions about best
  use of Word, Excel, etc
• suggestions are often good
• BUT distraction can displace information from
  STM, slow down user
• especially cute animations...
• Dont distract the user!

28
Long-term memory

• Large isnt exhausted even in 100 yrs
• Potentially permanent
• more permanent if rehearsed
• Retrieval time
• depends on recency of access
• 100s of ms for recently accessed info
• secs otherwise
• slowed if similar confusor

29
Example Passwords

• Dont use a password that is only used a few
  times per year (E-commerce?)
• likely forgotten because not rehearsed
• Dont use similar passwords
• spelling or usage context (eg, e-bookstore)
• likely to be confused, slow things down

30
General Lessons

• Dont distract the user
• cute animations may cause important info to be
  pushed out of short-term memory.
• Dont expect user to remember details for
  rarely-needed task or data.
• Be consistent with common tasks, data!

31
Conclusion

• Humans have cognitive limitations, strengths and
  weaknesses!
• Detailed knowledge is patchy but improving with
  research
• Good UIs respect these
• Most standard way of doing things respect them
  as well.