The principal task of the usability test moderator (or facilitator) is to conduct the test session with the participant. A key part of this involves presenting the tasks to be performed. The phrase "Define testing tasks" in this context refers to the moderator's role in explaining the scenario and communicating the specific goal of each task to the participant during the live session. While the tasks are designed beforehand by a test analyst, the moderator is responsible for defining them for the participant at the moment of execution.

B. Log usability problems: This is the primary responsibility of the observer (or note-taker), a distinct role designed to allow the moderator to focus entirely on interacting with and observing the participant.

C. Create a survey plan: This is a test design and planning activity that occurs before the test sessions begin and is typically the responsibility of the usability test analyst or test manager.

D. Write a usability test plan: This is a high-level strategic document created during the planning phase, and it is a principal task of the test manager, not the moderator.

1. ISTQB® Certified Tester, Usability Testing Syllabus (Version 2018):

Section 3.3.2, Roles in Usability Reviews and Testing: This section defines the moderator's role: "The moderator is the person who leads a usability test session... The moderator gives the test participant the tasks and may ask questions." This act of "giving" the tasks involves defining their scope and goal for the participant. The syllabus explicitly assigns other tasks to other roles: "The usability test analyst is responsible for developing the usability test cases (i.e., the tasks)...", "The observer’s main responsibility is to... log their behavior...", and "The test manager is responsible for... creating the usability test plan...".

2. Rubin, J., & Chisnell, D. (2008). Handbook of usability testing: How to plan, design, and conduct effective tests (2nd ed.). John Wiley & Sons.

Chapter 14, Facilitating a Test Session: This chapter details the facilitator's (moderator's) responsibilities during the session, which include "Briefing the participant on the procedure and their role" and "Reading the scenarios to the participant." This act of briefing and reading scenarios is the practical application of defining the tasks for the participant.

3. Dumas, J. S., & Redish, J. C. (1999). A practical guide to usability testing. Intellect books.

Chapter 11, Conducting the Usability Test: The authors describe the test administrator's (moderator's) role, stating, "The administrator presents each task to the user..." (p. 299). This presentation is a form of defining the task within the context of the test session, distinguishing it from the planning and logging activities.