Cognitive dimensions of notations

Cognitive dimensions or cognitive dimensions of notations are design principles for notations, user interfaces and programming language design, described by researchers Thomas R.G. Green and Marian Petre. The dimensions can be used to evaluate the usability of an existing information artifact, or as heuristics to guide the design of a new one.

Cognitive dimensions are designed to provide a lightweight approach to analysis of a design quality, rather than an in-depth, detailed description. They provide a common vocabulary for discussing many factors in notation, UI or programming language design. Also, cognitive dimensions help in exploring the space of possible designs through design maneuvers, changes intended to improve the design along one dimension.

User activities
The authors identify four main user activities with interactive artifacts: incrementation [creation], transcription, modification and exploratory design. Each activity is best served by a different trade-off in the usability on each dimension. For example, a high viscosity (resistance to change) is harmful for modification and exploration activities, but less severe for the one-off tasks performed in transcription and incrementation.

Design maneuvers
A design maneuver is a change in the notation design to alter its position within a particular dimension. Dimensions are created to be pairwise independent, so that the design can be altered in one dimension while keeping a second one constant.

But this usually results in a trade-off between dimensions. A modification increasing the usability of the notation in one dimension (while keeping a second one constant) will typically reduce its usability in a third dimension. This reflects an assumption in the framework that there is no perfect interface and that trade-offs are a fundamental part of usability design.

An example of a design maneuver is reducing the viscosity of a notation by adding abstraction mechanisms. Stylesheets are an example of a notation where this trade-off is made: a style sheet is an abstraction that represent the common styling attributes of items in a document. Changing the style sheet will modify all items at once, eliminating the repetition viscosity present in the need to change the style of each individual item.

List of the cognitive dimensions
Thomas Green originally defined 14 cognitive dimensions:


 * Abstraction gradient : What are the minimum and maximum levels of abstraction exposed by the notation? Can details be encapsulated?


 * Closeness of mapping : How closely does the notation correspond to the problem world?


 * Consistency : After part of the notation has been learned, how much of the rest can be successfully guessed?


 * Diffuseness / terseness : How many symbols or how much space does the notation require to produce a certain result or express a meaning?


 * Error-proneness : To what extent does the notation influence the likelihood of the user making a mistake?


 * Hard mental operations : How much hard mental processing lies at the notational level, rather than at the semantic level? Are there places where the user needs to resort to fingers or penciled annotation to keep track of what’s happening?


 * Hidden dependencies : Are dependencies between entities in the notation visible or hidden? Is every dependency indicated in both directions? Does a change in one area of the notation lead to unexpected side-effects?


 * Juxtaposability : Can different parts of the notation be compared side-by-side at the same time?


 * Premature commitment : Are there strong constraints on the order with which tasks must be accomplished?
 * Are there decisions that must be made before all the necessary information is available? Can those decisions be reversed or corrected later?


 * Progressive evaluation : How easy is it to evaluate and obtain feedback on an incomplete solution?


 * Role-expressiveness : How obvious is the role of each component of the notation in the solution as a whole?


 * Secondary notation and escape from formalism : Can the notation carry extra information by means not related to syntax, such as layout, color, or other cues?


 * Viscosity
 * Are there in the notation any inherent barriers to change? How much effort is required to make a change to a program expressed in the notation?
 * This dimension can be further classified into the following types:
 * 'Knock-on viscosity' : a change in the code violates internal constraints in the program, whose resolution may violate further internal constraints.
 * 'Repetition viscosity' : a single action within the user’s conceptual model requires many, repetitive device actions.
 * 'Scope viscosity' : a change in the size of the input data set requires changes to the program structure itself.


 * Visibility : How readily can required parts of the notation be identified, accessed and made visible?

Other dimensions
In addition to the above, new dimensions are sometimes proposed in the HCI research field, with different levels of adoption and refinement. These candidate dimensions are based on research outside the cognitive dimensions framework, and are adapted to it as a way to summarize that research into the hands-on approach encouraged by it.

Some of these candidate dimensions include creative ambiguity (does the notation encourage interpreting several meanings of the same element?), indexing (are there elements to guide finding a specific part?), synopsis ("gestalt view" of the whole annotated structure) or unevenness (some creation paths are easier than others, which bias the expressed ideas in a developed artifact).