GOMS ModelsDr. Yan LiuDepartment of Biomedical, Industrial and Human Factors EngineeringWright State University2Cognitive Models What are Cognitive Models Representations of some aspects of the user’s understanding, knowledge, intentions or processing The level of representation differs from technique to technique Models of high-level goals and the results of problem-solving activities Descriptions of motor-level activity (e.g. keystrokes and mouse clicks) Why are Cognitive Models If we can build a model of how a user works, then we can predict how he/she will interact with the interface3GOMS Models Four Components of Cognitive Structure Goals Operators Methods for achieving the goals Selection rules for choosing among competing methods Goals What the user wants to achieve Represent a “memory point” for the user, from which he/she can evaluate what should be done and to which he/she may return should any error occurs Decomposed into subgoals4Move sentenceSelect sentenceCut sentencePaste sentence5GOMS Models (Cont’d) Operators Basic actions (lowest-level actions) available for performing a task May affect the system e.g. Press a key on the keyboard May affect only the user’s mental state e.g. Read a dialog box There is a degree of flexibility about the granularity of operators Command level e.g. Issue the “select” command More primitive level e.g. Move mouse to menu bar, press the left mouse button, ….6GOMS Models (Cont’d) Methods Sequence of operators (procedures) for accomplishing the goalGoal: Select a sentenceMethod:Move mouse pointer to the first word of the sentencePress the left button of the mouseDrag the mouse to the last word the sentenceRelease the button7GOMS Model (Cont’d) Selection Rules Invoked when there is a choice of a method e.g. To cut a sentence, one can use the pull-down menu option, toolbar, or use CTRL+X shortcut GOMS attempts to predict which method will be used Depends on the particular user, the state of the system, and details about the goals. e.g. A user, Sam, always uses the icon on the toolbar to cut a sentence if the mouse is used. If the touchpad instead of mouse is used, he always uses CTRL+X to cut a sentenceUser Sam: Rule 1: If mouse is used, use the icon on the toolbar to cut a sentenceRule 2: If touchpad is used, use CTRL+X to cut a sentence8Construct GOMS Models Steps  Similar to hierarchical task analysis but in different notations Start with a high-level goal The goal is then decomposed into a sequence of unit tasks (subgoals) All the unit tasks are further decomposed down to the level of basic operators9Create a GOMS description of the task of photocopying an article from a journalGoal: Photocopy an article• Goal: Locate article• Find the pages of the article on the journalGoal: Photocopy pages … repeat until no more pages• Goal: Orient page• Open cover• Select page in the journal• Position page on the copier• Close cover• Goal: Copy page• Press copy button• Goal: Verify copy• Locate out tray• Examine copy• Goal: Collect copy• Locate out tray• Remove copy (Outer goal is satisfied)Goal: Retrieve the journalOpen coverRemove journalClose coverThe user could easily forget to retrieve the journal (closure problem)10Variants of GOMS Models Keystroke-Level Model (KLM) Card, Moran, and Newell GOMS (CMN-GOMS)  Cognitive-Perceptual-Motor GOMS (CPM-GOMS)11Keystroke-level model (KLM) Overview A simplified version of GOMS Execution time is estimated by listing the sequence operators and then summing the times of individual operators Aggregates all perceptual and cognitive functions into a single value for an entire task Does not employ selection rules  Seven Classes of Operators K: keystroking  0.12s for a good typist (90 wpm), 0.28s for a poor typist (40 wpm), 1.20s for non-typist B: pressing a mouse button  0.10s for down/up; 0.20 for clicking P: pointing with the mouse to a target on the display  Fitt’s law (0.1*log2(D/S+0.5)), 1.10 for average movement H: moving hands to the home position on the keyboard or mouse 0.40s D: drawing lines using the mouse Domain dependent M: Mentally preparing for a task  1.35s R: system response if the user waits for the system12Suppose we are using a mouse-based editor and noticing a single character error, so we will point at the error, delete the characters and retype it, and then return to our previous typing pointThis can be decomposed as follows:1. Move hand to the home position on mouse H[mouse]2. Position mouse after the wrong character P B[left]3. Move hand to keyboard H[keyboard]4. Delete character M K[DELETE]5. Type correct character K6. Reposition insertion point H[mouse] M P B[left]Texecute= TK+ TB+ TP+ TH+ TD+ TM+ TR= 2tK + 2tB+ tP+ 3tH+ 0 + 2tM+ 0= 2∙0.12 + 2∙0.2 + 1.10 + 3∙0.40 + 0 + 2∙1.35 + 0= 5.64s13CMN-GOMS Overview Proposed in Card, Moran & Newell (1983) The original GOMS model  Builds on KLM by adding subgoals and selection rules Can predict operator sequence as well as execution time Has been used to model word processors, CAD system for ergonomic design, and Sun Microsystem’s web page14A currently selected window can be closed to an icon (iconized) either by selecting the “minimize” option on the title bar of the window or by pressing “CTRL+F9” keys. Suppose a user, Sam, uses the “minimize” option to iconize a window when he has the mouse available; otherwise, he uses the “CTRL+F9” keysGoal: Iconize window• [Select Goal: Use title bar method …………if the mouse is available • Move mouse to the title bar of the window• Click the “minimize” option on the title barGoal: Use “CTRL+F9” method …….if the mouse is not available• Press “CTRL+F9” keys15Cognitive-Perceptual-Motor GOMS (CPM-GOMS) Overview  Builds on previous GOMS models by assuming that perceptual, cognitive and motor operators can be performed in parallel  It employs a schedule chart (also known as a PERT chart) to represent operators and dependencies between operators A PERT (program evaluation review technique) chart is a project management tool used to