Table 1.

Usability taxonomy (the intervention referred to in the taxonomy is the intervention itself as well as its embodiment, the user interface).

Usability attributes		Measures (example)
Learnability
	1. Consistency	The intervention is consistent in the use of interface aspects such as layout, buttons, and language (eg, the OK button is always on the left and the Cancel button is always on the right side, consistent use of terminology and look-and-feel)
	2. Conventions	The intervention follows platform conventions (eg, in Windows, the cross at the upper right corner of the screen is always used to close the window)
	3. Intuitiveness	The intervention characteristics intuitively imply its functionality and use (eg, a button with an arrow pointing to the right, implying “go to the next page”)
	4. Visibility of system status	The intervention provides feedback about its (future) state, action, and result (eg, when loading, the system provides a load bar showing how much time has passed and how much time remains)
Efficiency
	5. Flexibility	The intervention caters to a variety of users, both inexperienced and experienced (eg, the system provides both viewable icons, such as a floppy disk, and short-cuts, such as Ctrl-S)
	6. Structure	Using the intervention, users understand the structure of the intervention and know where they are (eg, the intervention provides breadcrumb navigation, ie, showing previous interaction steps and steps to come)
	7. Defaults	The intervention makes use of default settings (eg, fields containing defaults come up selected and the user can replace the default contents with new information—the defaults are user-specific)
Memorability
	8. Tailoring to user group	The intervention speaks the user groups’ language, with words, phrases and concepts familiar to the user group, rather than intervention-oriented terms (eg, intervention contains words and phrases fit for children, intervention uses read-aloud function for low literates)
	9. Recognition rather than recall	The intervention minimizes the user’s effort by making options visible or easily retrievable whenever appropriate (eg, the intervention shows context specific relevant available functionalities instead of referring to a manual where all options are listed)
Errors
	10. Error recovery	The intervention supports undo and redo (eg, the intervention offers a “Go” and a “Back” button)
	11. Error prevention by the system	The intervention prevents problems from occurring and notifies the user if a problem can potentially occur (eg, the intervention indicates which fields are mandatory [*] and applies form validation, such as the right format for postal code)
	12. Error recognition and resolution by the system	The intervention provides error messages expressed in plain language (no codes), which precisely indicate the problem and constructively suggest a solution (eg, when entering a faulty password, the intervention indicates: “Your password is incorrect, please ensure your CAPS LOCK key is off”)
	13. Help by the system	The intervention provides help information that is easy to search and focuses on the user’s task (eg, the intervention has a help function, in the form of a question mark icon, for every text field that needs to be entered)
Satisfaction
	14. Minimalistic	The intervention does not contain elements that are irrelevant or rarely needed (eg, the intervention interface is not cluttered, does not use distracting irrelevant interface elements, and does not require extensive scrolling)
	15. Aesthetic appearance	The intervention is esthetically attractive (eg, includes pictures, colors)
	16. Fun	The intervention is fun to use (eg, the intervention offers a challenge to users and arouses their curiosity)
	17. Modality integrity	The intervention offers information in a suitable modality (eg, information is presented in text when presenting details, information is presented in image when providing an overview)
	18. User control	The user is in control of the intervention (eg, the user initiates actions, the system justifies responses to actions of the user)
Personalization
	19. Adaptability by the user	The user can adapt the intervention to fit to their preferences and skill level (eg, the intervention offers the user to decrease and increase interface’s font size, the intervention offers the possibility to take a tour through the intervention or change the background color)
	20. Adaptiveness to the user	The intervention is aware of the user’s characteristics and adapts the interface to these characteristics (eg, the intervention is aware the user is farsighted and increases the font size)
	21. Adaptiveness to the context	The intervention is aware of the user’s context and adapts the interface to this context (eg, when the user is using the intervention in a public space, sound is turned off)
	22. Adaptiveness to the task	The intervention is aware of the task that the user aims to perform and adapts the interface to this task (eg, the intervention detects the user is making a presentation and the intervention provides the tools generally used, such as inserting text boxes and images)
Situatedness
	23. Prediction of behavior	The intervention accurately predicts the user’s behavior (eg, the intervention detects the user is in the office at 4 p.m. and predicts they will probably go home within the hour)
	24. Adaptation to behavior	The intervention accurately adapts the interface to the predicted behavior (eg, the intervention predicts they will go home within the hour and automatically offers a weather and traffic forecast)
Social interaction(these three questions apply to the automated system, and not to contact with a human person such as a therapist or health care professional)
	25. Embodiment	The online intervention appears humanlike (eg, the intervention has a face, eyes, and/or body that are used for non-verbal communication. This can be in the form of a robot, avatar, or character)
	26. Communication	The intervention applies humanlike communication skills (eg, the intervention expresses social thoughts, feelings, and behaviors)
	27. Following conventions	The intervention follows and applies social conventions (eg, the intervention applies turn taking, cooperation, and/or reciprocity)