I should write an experimental design dissertation based on ADHD and have at least 250 participants. Do you have any recommendations? How can I run a mixed-method design in this genre?
Your design is based on your research questions. For each research question, you should have a plan for data collection and analysis.
For example, if you wish to combine quantitative and qualitative data collection, some research questions should be about facts, such as "to what extent did...?"
Questions that will be answered by qualitative data should be about the experiences and perceptions of the participants.