gary marsdenslide 1university of cape town human-computer interaction - 6 user models gary marsden (...

Gary Marsden Slide 1University of Cape Town

Human-Computer Interaction - 6

User Models

Gary Marsden([email protected])

July 2002


Unit Objectives

We shall cover– GOMS– ICS

Rationale:– We need to reason about what types of tasks

users are good at and predict the impact of design decisions on the user

– User models allow us to do that without building prototypes for testing


What is User Modelling

Last time we looked at the tasks of the wider system

Now we want to look inside the user’s head to have some idea of how users are likely to interact with the system– We shall look at GOMS & ICS


GOMS

Stands for Goals, Operators, Methods and Selection rules

Derived from studying humans as information processors – Model Human Processor

Comprised of– Processors, Memories & Principles

Combine to form systems– Perceptual, cognitive, motor


MHP Overview


Back to GOMS

Goals are what people want to do– Like tasks in HTA

Operators are atomic perceptual, motor or cognitive acts which are necessary to change user’s mental state or environment

As such they are the lowest level of a GOMS analysis– Using GOMS a user’s behavior can be recorded

as a sequence of operators as operators can’t occur concurrently.


Example Operation

For example, to move a file to a different folder the user might perform the following operations:

– Move cursor to item– Hold mouse button down– Locate destination icon– Let go of mouse button


Methods

Sequence of operators: can have sub-methods Assumption is that methods are learnt and are

routine (no processing required) For example, a user moving a file to a different

folder could be described in GOMS as:– Goal – move file to a different folder

• Method – move file– Operators - Move cursor to item, Hold mouse button down, Locate destination icon, Let go of mouse button


Selection Rules

Used to select between alternative methods to achieve a given goal

Kind of like a bunch of if..then..else statements

Goal - print current document– IF (toolbar visible) THEN click toolbar button

– ELSE type CTRL-P


Keystroke Level Model

The lowest level of GOMS is called the KLM Used to predict task completion times for skilled

operators Each task described as 5 physical-motor operators:

– Tk: (k)eying – how long it takes to press a key (including using modifiers such as the shift key)

– Tp: (p)ointing – how long it takes to move the mouse (or other such input device) to a target

– Th: (h)oming – how long it takes to change between input devices e.g. changing between mouse and keyboard

– Td: (d)rawing – how long it takes to draw a line using an input such as a mouse

– Tb: click (b)utton – how long it takes to click the mouse button


Mental operators

Beside the motor operators there are– Tm: (m)ental operator – how long it takes to

perform the mental processing for the task– Tr: system (r )esponse operator – how long the

system takes to respond

Consider an example: we want to change– The quick fox jumps over the lazy dog

To– The quick brown fox jumps over the lazy dog


KLM example

1. move hand to mouse – H (mouse)

2. position mouse just after ‘quick’ – P,B

3. move hand to keyboard – H(keyboard)

4. formulate word to insert – M

5. type ‘brown’ – K (b) K (r) K (o) K (w) K (n) K ( )6. reposition insertion point at end of sentence – H

(mouse), M, P, BTotal time is: 3Th + 2Tp + 2Tb + 2Tm + 6Tk


Absolute Time

To calculate a real time, you can observe the users…

… or use the KLM estimates– Operators Time (s)

– Tk 0.12– Tp 1.10– Th 0.40– Td 1.06– Tb 0.20– Tm 1.35


GOMS & ATMs

– GOAL: GET-MONEY• GOAL: USE-ATM

– INSERT-CARD– ENTER-PIN– ENTER-AMOUNT– COLLECT-MONEY– COLLECT-CARD

Task completed at this point, so

users stop

Closure– Having achieved a task, your brain is satisfied

and clears up memory for the next task


GOMS summary

Designed for expert users performing routine tasks

Gives insight into time to complete taskCan prove very useful in the correct

situation– Telephone company

Not a widely applicable tool though


Interacting Cognitive Sub-systems

ICS is an elaborate framework which assumes that human perception, cognition, and action can be analysed in terms of discrete, inter-linked, information processing modules

In contrast to GOMS, ICS is a much richer way of modelling human cognition as we shall see in this section


ICS components

ICS assumes three independent sub-systems – sensory – visual and auditory stimulus– representational – representations of

information– effector – body movement

Each subsystem has– One input– One or more outputs– Memory– Transformations


Example - Perception

Here input (lines) is percievedTransformed to a recognised shapeShape stored in memory for later use


Full system

The real power comes from seeing the cognitive load placed on the user completing a task.


Summary

We have looked at – GOMS: a system for predicting expert user

performance– ICS: an example of a more complex user model

gary marsdenslide 1university of cape town human-computer interaction - 6 user models gary marsden (...

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