Cognitive Modelling (ACS)

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Cognitive Modelling

Limitations of the GOMS approach

Tends to focus on optimizing a task, the broader scope of usability is dificult to objectify

Best suited to procedural, routine tasks

Doesn't take into account environmental factors, interruptions, distractions, other cognitive loads or multitasking

Assumes there are reliable estimated times for components of tasks. Not always the case.But, pilot studies can be used to find these.

Must assume users are practised and make no mistakes (advanced user vs beginner)

Fitts Law

Increases with distance to targetDecreases with width of targetTime depends on relative precision

E.g Mozila X tab button

"Targets that are small/further away require more time to acquire"

Models Movement time for selection

Rational Analysis

CPM COMSCognitive perceptual-motoror Critical Path Method

Critical path is the sequence of operators with the longest overal duration.This determines shortest time to complete the task.

Assumes that Cognitive, perceptual & motor operators performed run in parallel.This is represented in a schedule chart ['PERT'].Perceptual top line. Cog middle line. Motor bottom line.

Theoretical basis is Model Human Processer, Card et al 1983

KLM GOMSKeystroke-Level Model

Different attributes of user or environment may change the time. e.g. expert vs beginner typists.

The 8 Standard Operators:B Press a key (0.28s)T(n) Type a sequence of n chars on keyboard (n x Ks)P Point the mouse to a target on the display (1.1s)B Press or release mouse button (0.1s)BB Click the mouse button (0.2s)H Move hands between mouse and keyboard (0.4s)M Mental act of routine thinking or perceptin (1.2s)W(t) Wait time for system response (t s)

All operators have an execution time.KLM-GOMS is mainly used for predicting time it takes to complete a task By the SUM of all operators.

Restricted ver of GOMS. Does not include Goals or Selection rules.States operators and methods to perform a task

Model Human Processor

Processros defined by cycle time

Memories defined by storage capacity, decay time, type of coding

ACT-R - A theory for simulating& understanding cognition

The second implementation of rational analysis is based on choosing between production rules.We optimise expected utility. ACT-R simulates this by giving each production an expected Probabilityand expected Cost. It is assumed with choose based on optimsing efficiency

Each chunk in declarative memory is based on the notion of activation.Each chunk has a level of activation which determines availability based on recency & frequency of use

E.g. children suming 3 + 4. count up? or recall it?

Cognition proceeeds via a pattern matching process that attempts to find production rules with certain conditions that match the current contents of the buffers. When a match is found, theproduction fires the action. These actions will be associated with appropriate latency.

Procedural memory, structured as Productions

How to do things

Declarative memory, structured as Chunks

things we know, e.g. facts

ACT-R Most popular. Builds on theory of rational analysis

Difference with GOMS (only expert), Cog Architectures can allow modeling of users learning the system

GOMS

Modelling number of actions (as opposed to just time)

E.g. Method for goal: delete object1. Accomplish goal: move object to trash2. return with goal accomplished

Can also use IF, AND, OR statements

Lends ideas taken from Task Analysis. Goals -> sub goals -> then one (or more) of the 4 items above

Predictive model. Supports evaluation before spending time building the system

Goals (aims of user)Operators (possible interactions e.g. clicking a mouse, slecting a key)Methods (Sequences of sub goals & operators to achieve a goal)Selection (rules by which a choice is made out of alternative options)

Representing the Brain

Models can be built to predict performance

A computational approach: Whaty is goal? How can it be computed in an algorithm.Input -> Internal representation (algorithm) -> output