The epidemiological unit measuring exposure and outcome (e.g. individual animals or herds). The term has the same meaning as ‘data unit’ in this context.

This occurs when individuals or groups in a study (the ‘study population’) differ systematically from the population of interest (the ‘target population’).

The actual population from which eligible study subjects are drawn.

The units selected from the source population.

An error that represents a consistent deviation from the true value. It can arise during the design, data collection, data management or analysis stages in surveillance systems or studies.