First, think about what the unit of replication is. This is not a
trivial as it sounds (a cage with 3 animals in it is not necessarily n = 3!). A
common mistake is to have all members of Group 1 in one cage, and all members
of Group 2 in another cage. The unit of replication
in this case would be cage, not individual (i.e., one cage would constitute n = 1!). The reason this is important is
because the variability within each
cage will usually be lower than the
variability between each cage. So,
any effect that you observe might be an effect of cage, rather than an effect
of your treatment.
Steps
to follow to ensure balanced allocation to treatment groups:
1.
Groups should be
age/sex matched as far as is possible.
2.
Groups should be
formed from multiple families (or original batches of animals), and randomly
allocated to treatment groups.
3.
If possible,
members of each group (e.g., 1 x Group 1 + 1 x Group 2, etc.) should share a
cage.
4.
Take care when
choosing marking substrates as some may interfere with behaviour. If possible, identify
individuals by natural markings.
5.
If possible,
include enrichment devices in cages, as overly standardized environments can adversely
affect the outcome of experiments.
6.
During the
experiment, ensure that all groups have equal exposure to handling/husbandry.
4.
Finally, it is a good idea to plan to carry out at least one (preferably more than one) independent replication of the study.