Any reduction in the incidence or severity of distress inflicted on the animals that are still necessary to use. Refinement represents the instances where the distress experienced by the individual animal from being used in a specific type of experiment is less than for previous runs of the same type of experiment. This welfare improvement can be achieved both by improving the procedures used as part of the experimental set-up or by improving the framework applying to the animal.
As is apparent in the definition of refinement, it overlaps with the principle of reduction, but refinement is directed more at actual experimental practice than the number of animals relative to the amount of knowledge. Existing methods can be refined with a view to increasing quality of life, for instance through pain relief or improved animal facilities that cater for the natural needs of the different species.
Another significant area of focus for the principle of refinement is the so-called humane endpoints that concern criteria for killing, aborting an experiment or pain treatment. Often, refinement can consist of an endpoint being defined as early symptoms of toxic poisoning or disease rather than letting the toxic poisoning or the disease play out.