Based on the concept of “conjecture and refutation” from the Popperian philosophy of science, i.e. looking for alternative causes, this book simplifies the design and inferences of human observational studies into two types: descriptive and causal. It clarifies how and why causal inference should be considered from the search for alternative explanations or causes, and descriptive inference from the sample at hand to the source population. Furthermore, it links the health policy and epidemiological concept with decisional questions, for which the basic measurement can be quality-adjusted survival time or quality-adjusted life year.