ARRIVE Essential - Study design

​​DISCLAIMER: Information on this and related pages is based on or copied directly from the ARRIVE guidelines 2019 (please see the original guidelines for more information, references and examples that are not included on these pages):

• updated guidelines for reporting animal research
• explanation and elaboration​

ARRIVE Essential 10 - Item 1 - Study design

For each experiment, provide brief details of study design including:

1a. The groups being compared, including control groups. If no control group has been used, the rationale should be stated.

The choice of control or comparator group is dependent on the experimental objective. Negative controls are used to determine if a difference between groups is caused by the intervention (e.g. wild-type animals vs genetically modified animals, placebo vs active treatment, sham surgery vs surgical intervention). Positive controls can be used to support the interpretation of negative results or determine if an expected effect is detectable.

It may not be necessary to include a separate control with no active treatment if, for example, the experiment aims to compare a treatment administered by different methods (e.g. intraperitoneal administration vs. oral gavage), or animals that are used as their own control in a longitudinal study. A pilot study, such as one designed to test the feasibility of a procedure might also not require a control group.​

For complex study designs, a visual representation is more easily interpreted than a text description, so a timeline diagram or flow chart is recommended. Diagrams facilitate the identification of which treatments and procedures were applied to specific animals or groups of animals, and at what point in the study these were performed. They also help to communicate complex design features such as clustering or nesting (hierarchical designs), blocking (to reduce unwanted variation), or repeated measurements over time on the same experimental unit (repeated measures designs).

The Experimental Design Assistant (EDA) is a platform to support researchers in the design of in vivo experiments, it can be used to generate diagrams to represent any type of experimental design.

Report the groups clearly so that test groups, comparators and controls (negative or positive) can be identified easily. State clearly if the same control group was used for multiple experiments.

1b. The experimental unit (e.g. a single animal, litter, or cage of animals).

The experimental unit is the biological entity subjected to an intervention independently of all other units, such that it is possible to assign any two experimental units to different treatment groups. The sample size is the number of experimental units per group.

Clearly indicate the experimental unit for each experiment so that the sample sizes and statistical analyses can be properly evaluated. There is a risk that if the experimental unit is not correctly identified, the sample size used in the data analysis will be incorrect. Inflation of the sample size by conflating experimental units with subsamples or repeated measurements is known as ‘pseudoreplication’. This may invalidate the analysis and resulting conclusions.

Commonly, the experimental unit is the individual animal, each independently allocated to a treatment group (e.g. a drug administered by injection). However, the experimental unit may be the cage or the litter (e.g. a diet administered to a whole cage, or a treatment administered to a dam and investigated in her pups), or it could be part of the animal (e.g. different drug treatments applied topically to distinct body regions of the same animal). Animals may also serve as their own controls receiving different treatments separated by washout periods; here the experimental unit is an animal for a period of time. There may also be multiple experimental units in a single experiment, such as when a treatment such as diet is given to a pregnant dam and then the weaned pups are allocated to different diets.

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