GOMS

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

Overview
GOMS reduces a user's interaction with a computer to its elementary actions (these actions can be physical, cognitive or perceptual). Using these elementary actions as a framework an interface can be studied. There are several different GOMS variations which 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. Operators are actions that are performed to get to the goal. Methods are sequences of operators that accomplish a goal. There can be more than one method available to accomplish a single goal, if this is the case then selection rules are used to describe when a user would select a certain method over the others. 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.

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, however it is also the most difficult GOMS technique to implement.

Success of GOMS
A successful implementation of CPM-GOMS was in Project Ernestine held by New England Telephone. New ergonomically designed workstations were compared to old workstations in terms of improvement telephone operators' performance. CPM-GOMS analysis estimated a 3% decrease in productivity. Over the four month trial 78,240 calls were analysed and it was concluded that the new workstations produced an actual 4% decrease in productivity. As the proposed workstation required less keystrokes than the original it was not clear from the time trials why the decrease occurred. However CPM-GOMS analysis made it apparent that the problem was that the new workstations did not utilize the workers' slack time. Not only did CPM-GOMS give a close estimate, but it provided more information of the situation.

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 unpredictability - such as user behaviour being affected by fatigue, social surroundings, or organizational factors. 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 work under the assumption that a user will know what to do at any given point - only applicable to expert users, novices are not considered.

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 (Cognitive Complexity Theory), or MHP (Model Human Processor). 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.

Software Tools
There exist various tools for the creation and analysis of Goms-Models. A selection is listed in the following:
 * GOMSED (Goms-Editor - in german)
 * QGoms (Quick-Goms)
 * CogTool KLM-based modelling tool