Clinical research of kidney diseases 1: researchable questions and valid answers.

Clinical epidemiology is the science of human disease investigation. It has a broad scope, ranging from the study of disease occurrence and prognosis to diagnostic and management options. This series of articles will discuss general and specific issues related to the design, analysis and interpretation of studies using nephrology examples. This first paper introduces general issues of ‘definition’ and ‘measurement’ in clinical studies, along with the related concepts of ‘reliability’ and ‘validity’, or ‘random’ error and ‘systematic’ error (bias).