An observation, over a given period, of a person, group or population defined at the outset, certain characteristics of whom or of which are evaluated to identify the changes in their health status or in certain health-related variables.

Note: If data on the effects on subjects of the intervention being studied are lost (because subjects have moved or have dropped out for some other reason), the study’s results may be influenced, especially if some types of subjects are systematically lost from the study for the same reasons. The investigators must report the number and type of subjects who could not be evaluated, in order that the possibility of bias may be taken into consideration.

Related concept: intention-to-treat analysis