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Replication in experimental design repeats a treatment on multiple independent experimental units, so that variation between those units can inform a treatment contrast and its standard error. Repeated measurements on one experimental unit can improve measurement precision without providing additional independent treatment replication. The experimental unit is identified by where the treatment is applied, rather than by how many responses are recorded.^[ref]

For a constructed example, grow one plant in each of eight separate pots. Apply treatment A to four pots and treatment B to four pots, without sharing drainage water between pots. Measure the length of three leaves per pot. The pot is the experimental unit; the leaf is the observational unit. There are four treatment replicates per group, eight experimental units overall, and $8\cdot 3=24$ measurements. The three leaves in one pot are nested measurements, because they share the same plant and treatment application. The setup assumes that pots do not affect one another; simply putting them in separate containers does not establish statistical independence.

Randomization chooses which experimental units receive which treatments; replication supplies multiple experimental units receiving the same treatment. These solve different problems. In the balanced allocation above, choose the four A pots uniformly among the $\left(\genfrac{}{}{0ex}{}{8}{4}\right)=70$ possible subsets. A particular pot has probability $1/2$ of receiving A. Given that one pot receives A, a second receives A with conditional probability $3/7$, rather than $1/2$. Thus the assignments are not independent random variables, even though the experimental units can supply independent responses conditional on those assignments in a suitable statistical model.

An actual ecology experiment illustrates why the distinction matters. In Stuart H. Hurlbert's account, twelve experimental ponds initially received zero, fifty or 450 fish, with five, four and three ponds respectively. Preparation included assessing variation between ponds before introducing fish; allocation considered both location and existing differences between ponds. The zero-fish ponds supplied the scientific control. These were repeated treatment applications to ponds, not twelve samples from one pond.^[ref]

The original article followed the southern California ponds for ten months. Its readouts included zooplankton populations and total phytoplankton; it reported decreases in some zooplankton populations and increased total phytoplankton.^[ref] Repeated sampling within a pond helps describe its response through time, while ponds supply treatment replication. Unequal group sizes and initial pond variation matter for the statistical model; the treatment counts alone do not establish equal variances or independent responses.

A parent-based treatment that targets accommodation and supportive responses to a child's anxiety. A randomised trial compared it with child CBT using prespecified noninferiority criteria; the tested age range and margins constrain the conclusion.^[ref]

Working memory training repeatedly practices tasks requiring temporary maintenance and manipulation of information, often adjusting difficulty as performance changes. Improving a practiced task, improving a different working memory task and improving classroom learning are separate outcomes.

A study using randomization of children with low working memory compared adaptive working memory training with comparison conditions and tested outcomes beyond the practiced exercises. Gains extended to some untrained working memory measures but did not establish improved academic progress or nonverbal reasoning.^[ref] This distinction motivates controls for practice and expectations, independent outcome tasks and follow-up measurements. A higher training score alone cannot show that a general cognitive capacity has increased; the claim depends on which additional tasks improve and for how long.

A randomised trial of a brief worry-focused intervention added to standard care for persistent persecutory delusions. The standard-care comparator permits inference about the added package, while unequal therapeutic contact limits a more ingredient-specific claim.^[ref]