[Principles of progression assessment-How can the progression of a disease be measured?]

The basic principles for the assessment of the progression of a chronic disease are universal. When they and their rules are understood, they can be used for different diseases and investigations. This article describes the different trajectories of chronic degenerative diseases and the possibilities for analysis of the progression are presented. The focus is on event-based and trend-based analyses The role of the main factors influencing the power of progression analysis, such as measurement errors and the number and timing of measurements are emphasized.