User Task Modeling

Approaches

• HF (prototype and test)
• Cognitive theory (production system)
• Engineering models (KLM and GOMS)

Keystroke Level Model (KLM)

• Choose representative user task scenarios
• Specify design to point that keystrokes defining actions can be listed
• List keystrokes (operators) required to perform task
• Insert mental operators at points user needs to stop and think
• Look up standard execution time for each operator
• Add up the execution times for the operators
• Total is estimated time to complete task

Standard Execution Times

• K - key press (0.2 sec = 55 wpm)
• P - point with mouse (1.1 sec)
• B - mouse button press (0.1 sec)
• H - home hands to keyboard or mouse (0.4 sec)
• M - mental act of thinking (1.2 sec)

See David Kieras' web pages for KLM sample tasks.

Placement of Mental Operators

• Hard to do - requires good intuition from designer
• Consistency in # of M's is more important than exact positioning

Tasks which require mental operators

• Initiating a task
• Making a strategy decision
• Retrieving a chunk from memory
• Finding something on the screen
• Verifying effectiveness of user action

Mental operators - New vs Experienced Users

• New users stop and check feedback after every step
• New users have small chunks
• Experienced users have elaborate chunks
• Experienced users may overlap mental operators with physical operators

GOMS Model

Goals Operators Methods Selection Rules

Advanatges

• GOMS models are executable
• GOMS models allow simulated execution of user task
• Provide a rigorous description of what user must learn
• Provide estimate of size or complexity of interface (number of distinct methods and their length)
• Can estimate both learning time (about 30 sec per step and execution time (total of KLM operators)
• Allow designer to evaluate the effect of reusing or sharing methods among several tasks

See David Kieras' web pages for GOMS sample tasks.

Method for GOMS Model Construction

• Make a list of top-level user goals
• Write a step-by-step method for accomplishing each goal on list
• Continue refining each step that is not a keystroke level operator by defining it as a subgoal and add it to the list of user goals
• Continue processing user goals until list is empty (meaning that all user goals are defined in terms of keystrokes)
• If there are multiple methods to accomplish a goal supply decision rules to choose which method to invoke

Method Definition

• List the steps needed to complete task (may be easier than listing goals)
• Methods longer than 5 steps are ignoring intermediate goal structure
• Normally each step should have a single keystroke operator

Identifying Selection Rules

• Needed when two or more methods can accomplish the same goal
• Hard to identify because it is rarely good to have multiple methods for same user goal
• Estimates of time requirements or mental operators can provide hints

GOMS Analysis Techniques

• Analyze a task as a class, not a task instances
• Task instances are like KLM task scenarios
• Typical mistake is to specify a KLM operator sequence as the GOMS model
• GOMS models should have 3 or 4 levels of goals before the specifying the keystroke operators
• Use a set of task scenarios to understand possible user goals
• Include tasks for the major methods and system functions
• Identify top-level, device independent goals
• Make judegement calls about task structure
• How users view task goals
• How users decompose tasks into subtasks
• What the natural order of steps in user methods is

Check and Revise the Method Analysis

• Test the GOMS model using hand or computer simulation
• Bypass complex cognitive task analyses
• • Use a high level operator as a place holder instead)
  • Assume needed information is present in task desciption and accessed as needed

Object-Action Interface Model (OAI)

• Focuses on task objects, interface objects, and interface actions
• Good correspondence between task and object decompositions will yield an interface that is easy to use by user familiar with task domain