Reduction
The main question here is:
“How can the number of animals used be reduced to the minimum needed to achieve the aim(s) of the work?”

This can be done in several ways:

• If related but different work has been done before, it can be used to assess the number of animals that will need to be studied to produce a definite result.
• Science-based statisticians can advise on the minimum numbers required.
• In some situations, the animals can be studied during spontaneous occurrences of the illness or injury or other phenomena of interest, so that no extra animals are required for the purposes of the study.
• Some studies can be done, painlessly and with no distress, in animals which have been studied for other scientific purposes. That can occur during a short period of unconsciousness while they are under anaesthetic just before they are euthanased using an overdose of that anaesthetic, or immediately after euthanasia.
• Tissues from animals killed in abattoirs can also be retrieved immediately after death and used for research, teaching or testing, thereby avoiding the need to use extra animals.