If I am doing a Mixed Method approach, Can I start the research by doing a questionnaire with students followed by interviews with lecturers ? or should I only do the interviews with students?
Your methods should match the research questions. What are they?
Do you need data from both students and lecturers? For which of your research questions?
In theory it could make sense to first do the student survey (if you have good established instruments to use) and then discuss survey results in the inverviews with the lecturers.
Also, it could make sense to first interview the lecturers to gain new insights and develop hypothesis, then use the interview results to construct the questionnaire of the survey to test the lecturer's assumptions.
But again, methods must follow your research questions.
Regarding your questions, Yes I need data from both students and lecturers.
"In theory it could make sense to first do the student survey (if you have good established instruments to use) and then discuss survey results in the inverviews with the lecturers"
Could you please share this theory, because when I search I found that:
The mixed-methods sequential explanatory design is starting with the questionnaire followed by interviews. But the point, in this theory, should I only select participants from the questionnaire sample or I can select different individuals?