GOMS

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GOMS stands for Goals, Operators, Methods, and Selection rules, an approach to human computer interaction observation developed by Stuart Card, Thomas P. Moran & Allen Newell, and spelled out in their book The Psychology of Human Computer Interaction, 1983. (ISBN 0-89859-859-1) Following these initial steps a whole family of engineering models for usability analysis evolved.

GOMS is essentially a reduction of a user's interaction with a computer to its elementary actions. Using these elementary actions as a framework, called the Keystroke-Level Model (KLM), an interface can be studied. Different GOMS variations allow for different aspects of an interface to be accurately studied and predicted.

For all of the variants, the definitions of the major concepts are the same. Goals are what the user intends to accomplish. An operator is an action performed in service of a goal. A method is a sequence of operators that accomplish a goal. There can be more than one method available to accomplish a goal. If more than one exists, then one of them is chosen by some selection rule. Selection rules are often ignored in typical GOMS analyses. There is some flexibility for the designers/analysts definition of all of these entities. For instance, one person's operator may be another’s goal. The level of granularity is adjusted to capture what the particular evaluator is examining.

1 Variations
2 Weakness of GOMS Overall
3 See also
4 References

[edit] Variations

The plain, or "vanilla flavored", GOMS first introduced by Card, Moran and Newell is now referred to as CMN-GOMS. Keystroke Level Modeling (KLM) is the next GOMS technique and was also introduced by Card, Moran and Newell in their 1983 book. This technique makes several simplifying assumptions that make it really just a restricted version of GOMS. The third major variant on the GOMS technique is the ‘Natural GOMS Language’ or NGOMSL. This technique gives a very strict, but natural, language for building GOMS models. The final variation of GOMS is CPM-GOMS. This technique is based on the Model Human Processor. The main advantage of CPM-GOMS is that it allows for the modelling of parallel information processing by the user.

[edit] CMN-GOMS Variation

This is the "vanilla flavored" GOMS originally introduced in Card, Moran and Newell’s book. This technique requires a strict goal-method-operation-selection rules structure. The structure is rigid enough that the evaluator represents the tasks in a pseudo-code format (no formal syntax is dictated). It also provides a guide for how to formulate selection rules. This method can also be used to estimate the load the task places on the user. For instance, examining the number of levels down the task-tree that a goal branch is can be used to estimate the memory demand the task places on the system. The process must remember information about all of the levels above the current branch. Card Moran and Newell were able to achieve 90% correct prediction rate using their method.

This technique is more flexible than KLM because the pseudo-code is in a general form. That is, it can be executed for different scenarios. KLM’s procedure is a simple list that has to be recreated for each slightly different task.

[edit] KLM-GOMS Variation

KLM is a simplified version of CMN-GOMS. It eliminates the goals, methods, and selection rules, leaving only primitive operators. Only six operations are provided by the base theory: 1) pressing a key, 2) moving the pointing device to a specific location, 3) pointer drag movements, 4) mental preparation, 5) moving hands to appropriate locations, and 6) waiting for the computer to execute a command. The times for each of the six operations have been empirically determined. The operations for a compete task are arranged into a serial stream, and total task execution time is a simple calculation.

This method assumes that operator times are invariant and do not depend on the previous sequence of events. New physical operators can be added if their timing can be represented as a simple context-free function. KLM-GOMS does not account for either slips or mistakes automatically -- the analyst must create separate models of error sequences and perform their own sensitivity analysis.

The placement of the keystrokes and pointer operations are straightforward, but the placement of the mental operations is not. Mental operations are placed by a set of rules that require some interpretation, such as determining a conceptual "cognitive unit" or grouping of actions. For instance, pressing ctrl and c simultaneously to perform a "copy" would be considered a single cognitive unit. Mental operations are inserted before each cognitive unit to account for cognitive preparation and decision-making.

The main reason a designer or analysts would use this technique is that it is a very fast. Different designs or systems can be compared against one another quickly. It does not require that the evaluator be an expert in GOMS because the procedure is an explicitly laid out recipe. The CogTool project has also developed an open-source tool to support KLM-GOMS analysis, improving the speed of analysis and helping to track results for multiple design variations. A major caution is that the algorithm is designed to estimate the execution time for an expert user, which is typically faster than the time for a new user or an unfamiliar task.

[edit] NGOMSL Variation

David Kieras developed the Natural GOMS Language technique in 1988. The motivation was to make GOMS/CCT (cognitive complexity theory) simple to use, similar to KLM, and still keep the power and flexibility of standard GOMS. This was necessary because GOMS did not have very well defined semantics. This lack of definition meant that two equally competent evaluators could do evaluations on the same system and come up with very different results. Kieras's result was the development of high level (natural language) syntax for GOMS representation with directions for doing a GOMS evaluation. The recipe is referred to as a "top down, breadth first" expansion. The user's high level goals are unfolded until only operators remain. Again, there are granularity considerations here. Generally operators are considered to be keystroke level operations but this is not a rigid requirement.

Since NGOMSL is based on CCT it has certain properties that make it unique. CCT gives estimations for execution times and predicts learning time. NGOMSL inherits this ability. It also, however, shares one of the major disadvantages the all of the previous methods. NGOMSL models user interaction as a serial operation. One operation occupies the user completely, there is no multitasking. This makes NGOMSL inappropriate for analyzing tasks where the users are under time pressure, highly practiced and, in reality, do act in a parallel fashion.

[edit] CPM-GOMS Variation

This work was done by Bonnie John, a former student of Allen Newell. She is considered one of the most influential researchers working in GOMS research. CPM stands for two things: Cognitive Perceptual Motor and the project planning technique Critical Path Method. Because it borrows elements of the critical path method, it is unique in that it does not make the assumption that the user's interaction is a serial process. The technique is also based directly on the model human processor. Evaluators begin a CPM-GOMS analysis in the same way they would a CMN-GOMS analysis. However, when the tasks are broken down just to the level where they are still perceptual or motor, the evaluator applies techniques from the model human processor. The tasks are first joined together serially and then examined to see which actions can be overlapped so that they happen in parallel. This technique facilitates representation of overlapping and very efficient "chunks" of activity characteristic of expert users. The estimated times by CPM-GOMS are generally faster since they do not allocate as much time to the "prepare for action" type operations.

This technique is the most difficult to implement. Therefore it has the problem of discrepancies between evaluators. Research is currently being conducted to improve the CPM-GOMS technique so that it can be used without the evaluator having a high level understanding of the GOMS theoretical foundations.

[edit] Weakness of GOMS Overall

All of the GOMS techniques provide valuable information, but they all also have certain drawbacks. None of the techniques address user fatigue. Over time a user's performance degrades simply because the user has been performing the same task repetitively. The techniques are very explicit about basic movement operations, but are generally less rigid with basic cognitive actions. It is a fact that slips cannot be prevented, but none of the GOMS models allow for any type of error. Further, all of the techniques are only applicable to expert users, novices are left out. Functionality of the system is not considered, only the usability. If functionality were considered, the evaluation could make recommendations as to which functions should be performed by the system (i.e. mouse snap). User personalities and habits are not accounted for in any of the GOMS models. All users are assumed to be exactly the same. Except for KLM, the evaluators are required to have a fairly deep understanding of the theoretical foundations of GOMS, CCT, or MHP. This limits the effective use of GOMS to large entities with the financial power to hire a dedicated human computer interaction (HCI) specialist or contract with a consultant with such expertise.

[edit] See also

Human information processor model

[edit] References

Judith Reitman Olson, Gary M. Olson: The Growth of Cognitive Modeling in Human-Computer Interaction Since GOMS, in: R. M. Baecker, J. Grudin, W. A. S. Buxton, S. Greenberg: Readings in Human-Computer Interaction: Towards the Year 2000. 1995, San Francisco, CA: Morgan Kaufmann.