Scientific knowledge includes two paths, deductive reasoning and induc tive reasoning. The former involves logical proofs and deductions and is not the subject of this volume. The latter involves drawing inferences from experiences and empirical data. Within the empirical method, there are both observational (sometimes called correlational) studies and experimental studies. Although both kinds of studies are essential to scientific progress, experimental studies generally play a greater role in the evaluation of interventions and therefore are the primary (although not exclusive) focus of this book.