Begin by assessing how well the student met the general requirements. Did he meet the allotted time, for example? Did he use the appropriate presentation software? If so, he met the basic requirements for the presentation and is eligible to pass if he did well on the contents of the presentation. If he did not meet these requirements, he fails the defense and, depending on your department's rules, he may have to defend the thesis again on a different date.
Assess the research in the thesis. As a committee member, you have read the student's thesis and commented on it. Did she revise according to your comments? Is the research well-designed, executed well and relevant to the field?
Assign a percentage to this area. A grade that is between 90 percent and 100 percent is excellent, or an A, and should indicate a superior job. A grade that is between 80 percent and 89 percent is good, or a B, and should indicate that the student more than met the minimal standards. A grade that is between 70 percent and 79 percent is adequate, or a C, and should indicate that the student met the minimum standards with no serious problems. A grade between 60 percent and 69 percent is minimally passing and had serious, but not failing, flaws, and is a D. A grade below 60 percent had fatal flaws and is an F.
Evaluate how well the student answered the questions at the end of the defense. Did he answer each question thoroughly and completely? Did he cite the appropriate sources in his response? How did he comport himself?: Was he professional even when he was presented with hostile or adversarial questions, if applicable? Assign a percentage grade to this area.
Assess the student's general presentation performance. While the performance is not as important as the quality of the research, answering questions and meeting the minimal assignment standards for the defense, she should be able to comport herself as a professional and speak with poise and confidence about her research. Assign a percentage grade to this area.
Consider the four elements of the presentation: meeting the presentation assignment criteria, research and revisions, answering questions and presentation skills. Your department might have its own rubric or weighting of these standards; if so, use the department rubric to assign a grade to the defense.
If your department does not assign a rubric for grading, use a weighted score. First, consider whether the student met the minimum requirements for the presentation. If she did, she is eligible to pass. If she did not, she fails automatically. The typical weighted scale assigns 50% of the final grade to research and revisions, 30% to answering questions and 20% to presentation skills. Calculate the percentage by multiplying each percentage grade by its weight, and then add the resulting three numbers together; this yields the final percentage grade. Assign a letter grade based on this weighting system.