1. Model based design

2. Objective
The interactive system design consisting of several stages such as
Requirement Specification
Design
Prototyping
Evaluation
The interactive system design is highly iterative

3. The process of design (recall from L15)
interviews
ethnography
what is there
vs.
what is wanted
guidelines
principles
standards
dialogue notations
precise specification
architectures
documentation
help
evaluation
heuristics
scenarios task analysis
what is wanted
analysis
design
implement
and deploy
prototype

4. Objective The iterative life cycle is
Time consuming and
Also requires cost (for coding and testing)
It is always good if we have an alternative method that reduces
Time, effort and cost required for the design life cycle
Model-based design provides one such alternative

5. Motivation Suppose you are trying to design an interactive system
First, you should identify such as
Requirements (“Know the user”)
Contextual Inquiry
Which are time and cost consuming and tedious process
Instead of going through the process, it would have been better if we have a “model of the user”
Which is known as model based design

6. Idea of a model A ‘model’ in HCI refers to
“A representation of the user’s interaction behavior under certain assumptions”
The representation is typically obtained form extensive empirical studies
Collecting and analyzing data from end users
The model represents behavior of average users, not individuals
It saves lots of time, cost and effort

7. Types of model 1. Descriptive/ prescriptive models
It describe user behavior during interaction in qualitative terms.
An example is the Norman’s model of interaction
These models help in formulating (prescribing) guidelines for interface design

8. Types of model 2. Predictive engineering models:
These models can “predict” behavior of a user in quantitative terms.
An example is the GOMS model (to be discussed later in this module)
GOMS model can predict the task completion time of an average user for a given system.

9. Predictive engineering models
The predictive engineering models used in HCI are of three types
Formal (system) model
Cognitive (user) model
Syndetic (hybrid) model

10. Predictive engineering models
Formal (system) models
The interactive system is represented using ‘formal specification’ techniques
For ex: ‘state transition networks’/’system state space’
Cognitive (user) models
These models capture the user’s thought (cognitive) process during interaction
For ex: A GOMS model tells us the series of cognitive steps involved in typing a word
Syndetic (hybrid) model
Complex and rarely used

11. Model based design limitations
The models can not capture individual user characteristics
It only models average user behavior

12. Summary
Model-based design reduce the need for real users in interactive system
However, they can not completely eliminate the role played by real users
It can be employed in the initial design stages

13. Descriptive/ prescriptive models: Donald Norman’s model
Seven stages
user establishes the goal
formulates intention
specifies actions at interface
executes action
perceives system state
interprets system state
evaluates system state with respect to goal
Norman’s model concentrates on user’s view of the interface