A variable is a concept or characteristic whose value or attribute changes or varies from one case to another, or across different individuals, groups, situations, or over time. It is a dimension along which an object, individual, or group can be categorised or measured, allowing for specific comparisons to be made with others or across different points in time. Examples of variables in sociological research include income level (which can vary from person to person), age (which changes over time for an individual), educational attainment (which differs among individuals), or social class (which can vary across groups or societies). Researchers examine the relationships between different variables to understand social phenomena, test hypotheses, and develop sociological theories. Variables can be independent (the presumed cause) or dependent (the presumed effect) in a causal relationship.