The interviewer is going to gauge your level of expertise in clinical trials based on the knowledge and confidence you display when answering this question. Keep your answer fact-related and easy to understand.
"Firstly, I believe it is important to not move onto the next phase of a clinical trial until the objectives of the current stage are met. In Phase 0, the most critical factor is to determine that the effect of the drug on the participant is as expected. In Phase I, evaluating the safety and proper dose is the most important factor. For Phase II..."
Entry Level answer example
"I have been taught that the most important factor in any phase of a clinical trial is the safety of your test subjects. Without a strong base of safety, no trial can be published without question."
Experienced answer example
"I have two ways of thinking through the start of the project. The first is excitement and curiosity for how the project may impact the world. The second is focused on the process and timeline of the clinical research plan. I think it's important to have the steps mapped out so that the team can stay on budget and on time."