Design Analysis Flashcards
Fitts’ law
can be used to determine the size and location of a screen object.
Fitts’ law states that the time it takes to hit a target is a function of the size of the target and the distance to that target
three parts to fitt’s law
ID - Index of Difficulty
* quantifies the difficulty of a task based on width and distance
MT - Movement Time
* quantifies the time it takes to complete a task based on the difficulty of the task (ID) and two empirically derived coefficients that are sensitive to the specific experimental conditions
IP Index of Performance
* based on the relationship between the time it takes to perform a task and the relative difficulty of the task
Fitts’ law - implications
overly elongated objects hold no advantage
objects should be elongated along the most common trajectory path
objects should not be offset from the screen edge
fitts law does not address touch specific problems (e.g. fat fingers)
Hick’s Law
The time T needed to make a decision is proportional to the log number of alternatives given
T= a+bH
H(Entropy) = log2(n+1)
a,b: coefficients, empirically determined from experimental design
requirements: no linear search, no power law of practice, equal probability
Power Law of Practice
Users get better every time they use a system
Time = B*N^-[alpha]
N: Trial Number
B, [alpha]: constants
Mental Models
People use a mental model to have a basic understanding of what is going on
The mental model is often unsharp
Mental models is a method to understand interaction that is both predictive and explanatory
- unscientific
- partial
- unstable
- inconsistent
- personal
Mental Models and Mapping
Mapping can be used to foster understanding and building a correct model - and thus increase usability
Two major principles of design
Mapping
- to a form
- to provide overview
Constraints
- to guide
- for support “affordance”
Mapping
correlation between control element and action
properties for good mapping
- understandable
- consistent
- recognizable or quickly learnable
- natural
Constraints
Constraints lead humans to build correct mental models
Constraints guide the user to the next appropriate action or decision
Constraints also minimize the chance to make errors
- physical constraints
- semantic constraints
- cultural constraints
- logical constraints
Affordance
To build a mental concept of a system we need to interpret symbols and components
This can be seen as the functionality of the device and what we actually want to do - The semantic and articulator Distance
3 principles of affordance
- make usable properties visible
- use natural associations
- give feedback
Norman’s Execution/Evaluation Action Cycle (EEAC)
Goals : we begin with some idea of what we want to happen
* Establish the goal
--> Execution: We must then execute and action * Forming intention * Specifying action * Executing action
–>
World/Action: Manipulate Objects in World
--> Evaluation: Finally, validate our action. compare results with goals * Evaluating Interpretation * Interpreting Perception * Perceiving World State
–> goals
Gulf of Execution
Mismatch between the user’s intentions and the allowable actions
Not enough knowledge about
- concept
- usage
- access to function
Good design minimizes the Gulf of Execution!
- how directly can the actions be accomplished?
- do the actions that can be taken in the system match the actions intended by the person?
Gulf of Evaluation
Mismatch between the system’s representation and the user’s expectations
Problems with
- comparing goal and state
- Interpretation of state
- Perception of state
Good design minimzes the Gulf of Evaluation
- Is the information about state of the system easily accessible?
- Is it represented to ease matching with intentions?
Norman’s principles of good design
- Stage and action alternatives should be always visible
- Good conceptual model with a consistent system image
- Interface should include good mappings that show the
relationship between stages
*Continuous feedback to the user
Critical points / failures
- Inadequate goal formed by the user
- User does not find the correct interface / interaction object
- User many not be able to specify / execute the desired action
- Inappropriate / mismatching feedback
Task Analysis
understanding people and how they carry out their work
Task analysis is the study of how work is achieved by tasks
Definition - Task
A task is a goal together with some ordered set of actions
Definition - Goal
A goal is a state of the application domain that a work system wishes to achieve. Goals are specified at particular levels of abstraction.
Definition - Action
An action is a task which has no problem solving associated with it and which does not include any control structure. Actions and task will be different for different people.
Cognition
thinking, solving problems, learning, memory and representations of things that people are assumed to have in their heads: mental models
Categories of task analysis methods
- concerned with logic of task : sequence of steps to achieve goal
- concerned with the cognitive aspects : understanding which cognitive processes the work system will have to undertake to achieve a goal
4 dimensions of task analysis
- notation
- usability for communication
- usability for modelling tasks
- adaptability to new types of systems, aims and requirements
Hierarchical Task Analysis (HTA)
graphical representation of a task structure
structure chart notation:
- sequence of tasks
- subtasks and actions as hierarchy
- actions can be repeated (iteration)
- alternative actions possible (selection)
- optional: annotations to indicate plans
not trivial:
- acquisition of task and subtask descriptions
- hierarchical modelling
- iterative process
HTA step-by-step
- Decide on the purpose of the analysis
- Define the task goals
- Data acquisition
- Acquire data and draft hierarchical diagram
- Recheck validity of decomposition with stakeholders
- Identify significant operations and stop when effects of failure are no longer significant
- generate and test hypotheses concerning factors affecting learning and performance
Goals, Operators, Methods, Selection Rules (GOMS)
An application of HIP and Task Analysis/Decomposition
Focusing on Cognitive Load Analysis
Explicit task structure
- hierarchy of goals and sub-goals
- Procedure: define goals and refine them
- Outcome are several sheets of GOMS descriptions, starting with the topmost description
- Methods in the topmost description are subsequently refined in the “lower” GOMS descriptions
Goals * user’s intentions (tasks)
Operators * actions to complete task
Methods * (subgoals) sequences of actions
Selections * rules for choosing appropriate method
Keyboard Level Model (KLM)
The KLM is a practical design tool that can capture and calculate the physical actions a user will have to carry out to complete specific tasks
The KLM can be used to determine the most efficient method and its suitability for specific contexts
KLM operators
K: press a key or button
P: point with a mouse
H: home hands to keyboard or peripheral device
(D: draw line segments)
M: Mental preparation
R: System response
