Glossary

Data that contain information about the sampled units (e.g. respondents, households) measured on two or more occasions.

Source: Learn Statistics Easily.

In long format data, each row represents a single observation or measurement for a subject, often resulting in multiple rows for each subject. For example, in a longitudinal survey tracking student performance over several years, each student will have multiple rows corresponding to different years, with each row recording their performance for that particular year. Contrast with wide-format data.

A longitudinal design is one that measures the characteristics of the same individuals on at least two, but ideally more, occasions over time. Its purpose is to directly address the study of individual change and variation. Longitudinal studies are expensive in terms of both time and money, but they provide many significant advantages relative to cross-sectional studies.

Source: SAGE Research Methods.