The debate between qualitative and quantitative research is a false dichotomy — the best researchers use both, strategically. This article explains when each approach is most valuable: qualitative research (interviews, observations, diary studies) for exploring 'why' and generating hypotheses; quantitative research (surveys, A/B tests, analytics) for testing hypotheses and measuring 'how much.' It provides a decision framework for choosing methods based on research questions, maturity of understanding, and available resources, with practical examples from product development and UX research.