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{{Short description|Form of human–computer interaction}}
In [[human–computer interaction]], the '''keystroke-level model''' ('''KLM''') predicts how long it will take an expert user to accomplish a routine task without errors using an interactive computer system.<ref>{{cite journal|last1=Card|first1=Stuart K|last2=Moran|first2=Thomas P|last3=Allen|first3=Newell|title=The keystroke-level model for user performance time with interactive systems|journal=Communications of the ACM|date=1980|volume=23|issue=7|pages=396–410|doi=10.1145/358886.358895|s2cid=5918086|ref=1|doi-access=free}}</ref> It was proposed by [[Stuart K. Card]], [[Thomas P. Moran]] and [[Allen Newell]] in 1980 in the ''[[Communications of the ACM]]'' and published in their book ''The Psychology of Human-Computer Interaction'' in 1983, which is considered as a classic in the HCI field.<ref>{{cite web|last1=Sauro|first1=Jeff|title=5 Classic Usability Books|url=http://www.measuringu.com/blog/usability-books.php|website=MeasuringU|accessdate=22 June 2015|ref=7}}</ref><ref>{{cite web|last1=Perlman|first1=Gary|title=Suggested Readings in Human-Computer Interaction (HCI), User Interface (UI) Development, & Human Factors (HF)|url=http://hcibib.org/readings.html|website=HCI Bibliography : Human-Computer Interaction Resources|accessdate=22 June 2015|ref=8}}</ref> The foundations were laid in 1974, when Card and Moran joined the [[PARC (company)|Palo Alto Research Center]] (PARC) and created a group named Applied Information-Processing Psychology Project (AIP) with Newell as a consultant aiming to create an applied psychology of human-computer interaction.<ref>{{cite book|last1=Card|first1=Stuart K|last2=Moran|first2=Thomas P|last3=Newell|first3=Allen|title=The Psychology of Human-Computer Interaction|date=1983|publisher=L. Erlbaum Associates Inc|___location=Hillsdale|isbn=978-0898592436|pages=ix–x|ref=4|url=https://archive.org/details/psychologyofhuma00stua}}</ref> The keystroke-level model is still relevant today, which is shown by the recent research about mobile phones and touchscreens (see [[#Adaptions|Adaptions]]).
== Structure of the keystroke-level model ==
The keystroke-level model consists of six operators: the first four are physical motor operators followed by one mental operator and one system response operator:<ref>{{cite journal|last1=Card|first1=Stuart K|last2=Moran|first2=Thomas P|last3=Newell|first3=Allen|title=The keystroke-level model for user performance time with interactive systems|journal=Communications of the ACM|date=1980|volume=23|issue=7|pages=398–400|doi=10.1145/358886.358895|s2cid=5918086|ref=5|doi-access=free}}</ref>
* '''K''' (keystroke or button press): it is the most frequent operator and means keys and not characters (so e.g. pressing SHIFT is a separate K operation). The time for this operator depends on the motor skills of the user and is determined by one-minute typing tests, where the total test time is divided by the total number of non-error keystrokes.
* '''P''' (pointing to a target on a display with a mouse): this time differs depending on the distance to the target and the size of the target,<ref>{{cite journal|last1=Fitts|first1=Paul M|title=The information capacity of the human motor system in controlling the amplitude of movement|journal=Journal of Experimental Psychology: General|date=1992|volume=47|issue=3|doi=10.1037/h0055392|ref=3|pmid=13174710|pages=381–91|s2cid=501599 }}</ref> but is held constant. A mouse click is not contained and counts as a separate K operation.
* '''H''' (homing the hand(s) on the keyboard or other device): This includes movement between any two devices as well as the fine positioning of the hand.
* '''D''' (drawing (manually) n<sub>D</sub> straight-line segments with a total length of D(n<sub>D</sub>, l<sub>D</sub>) cm): where n<sub>D</sub> is the number of the line segments drawn and l<sub>D</sub> is the total length of the line segments. This operator is very specialized because it is restricted to the mouse and the drawing system has to constrain the cursor to a .56 cm grid.
* '''M''' (mentally preparing for executing physical actions): denotes the time a user needs for thinking or decision making. The number of Ms in a method depends on the knowledge and skill of the user. Heuristics are given to help decide where an M should be placed in a method. For example, when pointing with the mouse a button press is usually fully anticipated and no M is needed between both operators.<ref>{{cite journal|last1=Card|first1=Stuart K|last2=Moran|first2=Thomas P|last3=Newell|first3=Allen|title=The keystroke-level model for user performance time with interactive systems|journal=Communications of the ACM|date=1980|volume=23|issue=7|pages=400–401|doi=10.1145/358886.358895|s2cid=5918086|ref=5|doi-access=free}}</ref> The following table shows the heuristics for placing the M operator:<ref>{{cite journal|last1=Card|first1=Stuart K|last2=Moran|first2=Thomas P|last3=Newell|first3=Allen|title=The keystroke-level model for user performance time with interactive systems|journal=Communications of the ACM|date=1980|volume=23|issue=7|pages=400|doi=10.1145/358886.358895|s2cid=5918086|ref=5|doi-access=free}}</ref>
{| class="wikitable collapsible" border="1"
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* '''R''' (response time of the system): the response time depends on the system, the command and the context of the command. It is only used when the user actually has to wait for the system. For instance, when the user mentally prepares (M) for executing
The following table shows an overview of the times for the mentioned operators as well as the times for suggested operators:
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| K || total typing test time/total number of non-error keystrokes<br />
Guidelines:<ref name="klm-paper-operators-table">{{cite journal|last1=Card|first1=Stuart K|last2=Moran|first2=Thomas P|last3=Newell|first3=Allen|title=The keystroke-level model for user performance time with interactive systems|journal=Communications of the ACM|date=1980|volume=23|issue=7|pages=399|doi=10.1145/358886.358895|s2cid=5918086|ref=5|doi-access=free}}</ref><ref name="klm-book-operators-table">{{cite book|last1=Card|first1=Stuart K|last2=Moran|first2=Thomas P|last3=Newell|first3=Allen|title=The Psychology of Human-Computer Interaction|date=1983|publisher=L. Erlbaum Associates Inc|___location=Hillsdale|isbn=978-0898592436|pages=[https://archive.org/details/psychologyofhuma00stua/page/264 264]|ref=4|url=https://archive.org/details/psychologyofhuma00stua/page/264}}</ref><br />
.08 (135 wpm: best typist)<br />
.12 (90 wpm: good typist)<br />
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| H || 0.4<ref name="klm-paper-operators-table" /><ref name="klm-book-operators-table" />
|-
| D || .9n<sub>D</sub> + {{not a typo|. 16 l}}<sub>D</sub><ref name="klm-paper-operators-table" /><ref name="klm-book-operators-table" />
|-
| M || 1.35<ref name="klm-paper-operators-table" /><ref name="klm-book-operators-table" />
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| colspan="2" | suggested operators
|-
| B (mouse button press or release) || 0.1<ref>{{cite web|last1=Kieras|first1=David|title=Using the Keystroke-Level Model to Estimate Execution Times|url=http://www-personal.umich.edu/~itm/688/KierasKLMTutorial2001.pdf|accessdate=22 June 2015|ref=9|page=2|archive-date=19 March 2015|archive-url=https://web.archive.org/web/20150319195627/http://www-personal.umich.edu/~itm/688/KierasKLMTutorial2001.pdf|url-status=dead}}</ref>
|-
| Click a Link/ Button || 3.73<ref name="sauro-suggested-operators">{{cite book|last1=Sauro|first1=Jeff|
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| Pull-Down List (No Page Load) || 3.04<ref name="sauro-suggested-operators" />
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== Advantages ==
The KLM was designed to be a quick and easy to use system design tool, which means that no deep knowledge about [[psychology]] is required for its usage.<ref name="klm-paper-advantages">{{cite journal|last1=Card|first1=Stuart K|last2=Moran|first2=Thomas P|last3=Newell|first3=Allen|title=The keystroke-level model for user performance time with interactive systems|journal=Communications of the ACM|date=1980|volume=23|issue=7|pages=409|doi=10.1145/358886.358895|s2cid=5918086|ref=19|doi-access=free}}</ref> Also, task times can be predicted (given the [[#Limitations|limitations]]) without having to build a [[prototype]], recruit and test users, which saves time and money.<ref>{{cite book|last1=Sauro|first1=Jeff|
== Limitations ==
The keystroke-level model has several restrictions:
* It measures only one aspect of performance: time,<ref>{{cite journal|last1=Card|first1=Stuart K|last2=Moran|first2=Thomas P|last3=Newell|first3=Allen|title=The keystroke-level model for user performance time with interactive systems|journal=Communications of the ACM|date=1980|volume=23|issue=7|pages=400|doi=10.1145/358886.358895|s2cid=5918086|ref=21|doi-access=free}}</ref> which means execution time and not the time to acquire or learn a task <ref>{{cite book|last1=Card|first1=Stuart K|last2=Moran|first2=Thomas P|last3=Newell|first3=Allen|title=The Psychology of Human-Computer Interaction|date=1983|publisher=L. Erlbaum Associates Inc|___location=Hillsdale|isbn=978-0898592436|pages=[https://archive.org/details/psychologyofhuma00stua/page/260 260–261]|ref=22|url=https://archive.org/details/psychologyofhuma00stua/page/260}}</ref>
* It considers only expert users. Generally, users differ regarding their knowledge and experience of different systems and tasks, motor skills and technical ability <ref name="klm-paper-limitations-users">{{cite journal|last1=Card|first1=Stuart K|last2=Moran|first2=Thomas P|last3=Newell|first3=Allen|title=The keystroke-level model for user performance time with interactive systems|journal=Communications of the ACM|date=1980|volume=23|issue=7|pages=397, 409|doi=10.1145/358886.358895|s2cid=5918086|doi-access=free}}</ref>
* It considers only routine unit tasks <ref name="klm-paper-limitations">{{cite journal|last1=Card|first1=Stuart K|last2=Moran|first2=Thomas P|last3=Newell|first3=Allen|title=The keystroke-level model for user performance time with interactive systems|journal=Communications of the ACM|date=1980|volume=23|issue=7|pages=409|doi=10.1145/358886.358895|s2cid=5918086|ref=24|doi-access=free}}</ref>
* The method has to be specified step by step.<ref name="klm-paper-limitations" /> This makes it more accessible to use for an average person without advanced technical skills.
* The execution of the method has to be error-free <ref name="klm-paper-limitations" />
* The mental operator aggregates different mental operations and therefore cannot model a deeper representation of the user’s mental operations. If this is crucial, a GOMS model has to be used (e.g. model K2)<ref>{{cite book|last1=Card|first1=Stuart K|last2=Moran|first2=Thomas P|last3=Newell|first3=Allen|title=The Psychology of Human-Computer Interaction|date=1983|publisher=L. Erlbaum Associates Inc|___location=Hillsdale|isbn=978-0898592436|pages=[https://archive.org/details/psychologyofhuma00stua/page/285 285–286]|ref=25|url=https://archive.org/details/psychologyofhuma00stua/page/285}}</ref>
Also, one should keep in mind when assessing a computer system that other aspects of performance (errors, learning, functionality, recall, concentration, fatigue, and acceptability),<ref>{{cite journal|last1=Card|first1=Stuart K|last2=Moran|first2=Thomas P|last3=Newell|first3=Allen|title=The keystroke-level model for user performance time with interactive systems|journal=Communications of the ACM|date=1980|volume=23|issue=7|pages=396–397|doi=10.1145/358886.358895|s2cid=5918086|ref=26|doi-access=free}}</ref> types of users (novice, casual)<ref name="klm-paper-limitations-users" /> and non-routine tasks have to be considered as well.<ref name="klm-paper-limitations-users"/>
Furthermore, tasks which take more than a few minutes take several hours to model and a source of errors is forgetting operations.<ref>{{cite book|last1=Sauro|first1=Jeff|
In addition, the KLM can not make a perfect prediction and has a root-mean-square error of 21%.<ref>{{cite book|last1=Card|first1=Stuart K|last2=Moran|first2=Thomas P|last3=Newell|first3=Allen|title=The Psychology of Human-Computer Interaction|date=1983|publisher=L. Erlbaum Associates Inc|___location=Hillsdale|isbn=978-0898592436|pages=[https://archive.org/details/psychologyofhuma00stua/page/275 275]|ref=29|url=https://archive.org/details/psychologyofhuma00stua/page/275}}</ref>
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{| class="wikitable"
|-
! Design A: drag the file into the trash can<ref>{{cite web|last1=Kieras|first1=David|title=Using the Keystroke-Level Model to Estimate Execution Times|url=http://www-personal.umich.edu/~itm/688/KierasKLMTutorial2001.pdf|accessdate=22 June 2015|ref=31|page=3|archive-date=19 March 2015|archive-url=https://web.archive.org/web/20150319195627/http://www-personal.umich.edu/~itm/688/KierasKLMTutorial2001.pdf|url-status=dead}}</ref> !! Design B: use the short cut “control + T”<ref>{{cite web|last1=Kieras|first1=David|title=Using the Keystroke-Level Model to Estimate Execution Times|url=http://www-personal.umich.edu/~itm/688/KierasKLMTutorial2001.pdf|accessdate=22 June 2015|ref=31|page=6|archive-date=19 March 2015|archive-url=https://web.archive.org/web/20150319195627/http://www-personal.umich.edu/~itm/688/KierasKLMTutorial2001.pdf|url-status=dead}}</ref>
|-
| method encoding (operator sequence)<ref>{{cite web|last1=Kieras|first1=David|title=Using the Keystroke-Level Model to Estimate Execution Times|url=http://www-personal.umich.edu/~itm/688/KierasKLMTutorial2001.pdf|accessdate=22 June 2015|ref=32|page=9|archive-date=19 March 2015|archive-url=https://web.archive.org/web/20150319195627/http://www-personal.umich.edu/~itm/688/KierasKLMTutorial2001.pdf|url-status=dead}}</ref>|| method encoding (operator sequence)<ref>{{cite web|last1=Kieras|first1=David|title=Using the Keystroke-Level Model to Estimate Execution Times|url=http://www-personal.umich.edu/~itm/688/KierasKLMTutorial2001.pdf|accessdate=22 June 2015|ref=33|page=10|archive-date=19 March 2015|archive-url=https://web.archive.org/web/20150319195627/http://www-personal.umich.edu/~itm/688/KierasKLMTutorial2001.pdf|url-status=dead}}</ref>
|-
|
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The six operators of the KLM can be reduced, but this decreases the accuracy of the model. If this low of an accuracy makes sense (e.g. “back-of-the-envelope” calculations) such a simplification can be sufficient.<ref>{{cite book|last1=Card|first1=Stuart K|last2=Moran|first2=Thomas P|last3=Newell|first3=Allen|title=The Psychology of Human-Computer Interaction|date=1983|publisher=L. Erlbaum Associates Inc|___location=Hillsdale|isbn=978-0898592436|pages=[https://archive.org/details/psychologyofhuma00stua/page/296 296]|ref=34|url=https://archive.org/details/psychologyofhuma00stua/page/296}}</ref>
While the existing KLM applies to desktop applications, the model might not fulfill the range of mobile tasks,<ref>{{cite book|last1=Li|first1=Hui|last2=Liu|first2=Ying|last3=Liu|first3=Jun|last4=Wang|first4=Xia|last5=Li|first5=Yujiang|last6=Rau|first6=Pei-Luen Patrick|title=Extended KLM for mobile phone interaction: a user study result|journal=CHI EA '10 CHI '10 Extended Abstracts on Human Factors in Computing Systems|date=2010|publisher=ACM|___location=New York|isbn=978-1-60558-930-5}}</ref> or as Dunlop and Cross <ref>{{cite journal|last1=Dunlop|first1=M.|last2=Crossan|first2=A.|title=Predictive Text Entry Methods for Mobile Phones|journal=Personal Technologies|date=2000|volume=4|issue=2–3|pages=134–143|doi=10.1007/BF01324120|s2cid=194691|url=https://strathprints.strath.ac.uk/2580/6/strathprints002580.pdf}}</ref> declaimed KLM is no longer precise for mobile devices. There are various efforts to extend the KLM regarding the use for mobile phones or touch devices. One of the significant contributions to this field is done by Holleis, who retained existing operators while revisiting the timing specifications. Furthermore, he introduced new operators: Distraction (X), Gesture (G), Initial Act (I).
While Li and Holleis <ref>{{cite book|last1=Holleis|first1=P.|last2=Otto|first2=F.|last3=Hussmann|first3=H.|last4=Schmidt|first4=A.|title=
Rice and Lartigue <ref>{{cite book|last1=Rice|first1=A.D.|last2=Lartigue|first2=J. W.|title=
They retain the operators Keystroke (K/B), Homing (H), Mental (M) and Response Time (R(t)) and suggest new touch specific operators partly based on Holleis’ suggested operators:
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