Sample vs Population

Case

A spatially and temporally delimited phenomenon of theoretical interest. For example, a country, the social media posts by a political party, or a survey respondent. Typically contains multiple observations

Observation

The lowest-level unit in an analysis at which a measured variable can only take one value (à unit of analysis). For example, a country-year, an individual post on social media, or a survey respondent. May or may not be of theoretical interest.

sample

The set of cases or observations that are analyzed in a given piece of research. For example, all European countries during a given time period, all social media posts by the Democrats and Republicans, all individuals in the UK.

population

The set of cases which in combination make up the universe of cases, i.e., all cases. For example, all countries in the world during a given time period, all social media posts by political parties, all individuals in the UK

what is population inference?

ferring something we do not know (patterns in the population) from something we do know (patterns in the sample)