What is defined as a special kind of systematic observation, used by correlational researchers,

that involves obtaining measures of the variables of interest in multiple waves over time.

Longitudinal design is defined as a special kind of systematic observation, implemented by correlational researchers, which involves the tasks of obtaining measures of variables of interest in multiple waves over time.

Explanation:

Longitudinal design is one of the known systematic observation pattern followed by the correlational researchers in which a subject of experiment is observed for a significantly long span.

The three types of longitudinal design are: panel study, cohort study and retrospective study. The complete design involves the multiple set of observations over a variable or subject. Longitudinal design is often used in clinical psychology to study rapid fluctuations in the behaviour.