A study can report many measurements from relatively few independently assigned units. Before repeating its sample size, identify what the authors counted as an experimental unit. The number of cells, images or repeated observations may answer a different question.
ARRIVE’s explanation of study-design item 1b describes the experimental unit through independent assignment and discusses units such as an animal, litter or cage. It warns that treating subsamples or repeated measurements as independent units can inflate sample size. Read the reported design before interpreting a number labelled n.
A fictional reading exercise
Imagine a report explicitly identifying cages as the independently assigned units, with three cages and four animals per cage in one group. The group contains twelve animals and three stated assignment units. Additional observations on those animals do not create additional independently assigned cages. This is a teaching example, not a report of a real experiment or a proposal to conduct one.
Do not apply that example automatically to every cage-based study. The relevant unit depends on the actual design, which may be more complex than a short summary suggests.
Keep three counts distinct
In an extraction note, record the stated experimental unit, number of those units per group and number of measurements or subsamples. Add a source location for each. If the paper uses several experiments, identify which experiment each count belongs to.
Where an analysis is unclear, preserve the question for statistical review. A reader should not retrofit an independent-observations assumption to justify a convenient headline number.
Limit the interpretation
Correctly identifying a unit is one part of understanding a study’s design. It does not establish that the methods are sound overall or that an animal finding applies to people. A large measurement count is not a shortcut to a clinical conclusion.
No animal study was appraised here, and no animal procedure is described. The practical task is a source note whose sample-size nouns match the report’s actual design.
Primary reporting guidance read 21 October 2026. Suggested note structures and explicitly fictional examples are editorial. No clinical result is evaluated.
Source: ARRIVE2.0 study-design item1b experimental unit. Related discussion: health information source quality.
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