Keystroke Level Model

1
Keystroke Level Model

• Professor L. A. Rowe
• April 28, 1999

2
Keystroke Level Model

• Card, Moran, NewellCACM, July 1980
• Outline
• Problem statement
• Model
• Empirical validation
• Applications

3
Problem Statement

• Goal
• Develop simple model to describe time to do task
  with a given method on an interactive system
• Ttask Tacquire Texecutewhere Ttask total
  time to complete task Tacquire time to select
  method to complete task Texecute time to
  perform method
• Model predicts Texecute
• Assume expert users and no errors

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Model

• Texecute ?(time to execute primitive op)
• Primitive operations
• K key/button press
• P point to target with mouse
• H home hands to keyboard or mouse
• D draw line with mouse
• M mental preparation (pause)
• R system response time

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Model (cont.)

• Times for primitive operations are predicated
  from experiments
• Time to press key/button ranges between
• 0.08 sec/char fast typist
• 0.28 sec/char average typist
• 1.2 sec/char slow typist

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Actual Parameters
D distance S size .8, 1.5
nD segments lD total length
values derived from experiments
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Encoding Method

• Code method as sequence of primitive operations,
  sum up times
• Example replace 5 letter word with another
• Editor 1 keyboard based
• next line MK(line feed)
• substitute command MKs
• enter new word 5Kxxxxx
• terminate arg MK(return)
• enter old word 5Kzzzzz
• terminate arg MK(return)
• terminate command K(return)
• Texecute 4tM 15tK 8.4 sec

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Encoding Method (cont.)

• Editor 2 mouse based
• grab mouse Hmouse
• point to word Pword
• select word Kyellow mouse button
• home to keyboard Hkeyboard
• replace command MKR
• type new word 5Kzzzzz
• terminate type in MK(esc)
• Texecute 2tM 8tK 2tH tP 6.2 sec
• Model Editor 2 is 2 secs (i.e. 25) than editor 1

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Placement of Ms

• rule 0 place M before all Ks not part of arg
  strings and before all Ps that select commands
• for each M do

rule 1 if operator fully anticipated, delete
M(e.g. PMK ? PK) rule 2 if string of MKs
belongs to cognitive unit, delete all Ms but
first rule 3 if K is redundant terminator,
delete M rule 4 if K terminates constant string
(i.e. command), delete itif K terminates
variable string (i.e. arg), keep it
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Placement of Ms

• Example Mouse-based Editor 2 sequence
• HmPwordKyHkKR5KK(esc)
• ? HmPwordMKyHkMKR5KMK(esc)
• ? HmPwordKyHkKR5KK(esc)

rule 0
rule 1
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Placement of Ms
Example Keyboard-based Editor 1
sequence K(lf)Ks5KK(ret)5KK(ret)K(
ret) ? MK(lf)MKs5KMK(ret)5KMK(ret)
MK(ret) ? MK(lf)MKs5KMK(ret)5KMK
(ret)K(ret)
rule 0
rule 3
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Empirical Validation

• Conducted experiment on real systems to compare
  predictions to observations
• 14 tasks on 3 types of systems
• text editors
• graphics systems
• command executers
• Users
• practice using system
• were tested to determine typing and drawing speed

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Empirical Validation

• Model predicts within 20 of average time to
  complete task
• 0.5 secs ? Tacquisition ? 4 secs

Text editors
Graphics editors
Observed time
Predicted time
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Why is KLM Useful?

• Provides model to reason about keystrokes
  required to implement operation
• Reminds us to think about timer required for
  basic interface actions
• Mental processing (M) is significant

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Applications

• Benchmark existing systems
• Develop sample tasks, compare editors
• Results
• POET SOS DISPED
• predicted 100 84 45
• actual 100 93 46

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Parametric Analysis

• Compare methods to accomplish task
• Example
• 2 methods to correct spelling error in word which
  is N words behind the current cursor position

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Parametric Analysis

• Method 1
• erase back to word, correct it, retype text
• next line MK(ctrl)
• substitute command n((1/c)MKw)
• enter new word 5.5K
• terminate arg 5.5(n1)K
• Texecute ? (1 n/c)tM (1 6.5)tK
• 1.6 2.16n secs
• Notes
• 1. c is chars per chunk ? 4
• 2. average letters per word ? 5.5

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Parametric Analysis

• Method 2
• exit type-in mode, replace word, re-enter type-in
  mode
• terminate type-in MK(esc)
• point/select error word Hmouse Pword
  Kyellow button
• invoke replace command Hkeyboard MKR
• type new word 4.5K
• terminate replace MK(esc)
• point/select type-in Hmouse Pword Kyellow
  button
• reenter type-in HkeyboardKI
• Texecute ? 4tM 10.5)tK 4tH 2tP
• 12.1 secs

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Parametric Analysis

• Method 3
• Backskip/resume commands
• set up backskip MK(ctrl)
• execute n 1 backskips (n 1)(1/c)MKS
• back word MKW
• type new word 4.5K
• execute resume M2K(ctrl-R)
• Texecute ? 3 (n 1)/c)tM (n 7.5)tK
• 5.8 0.62n secs

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Parametric Analysis

• Compare by plotting

15
Erase method (1)
Replace method (2)
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Texecute (secs)
Backskip/resume method (3)
5
n (words)