Randomized or fixed order for studies of behavioral syndromes?

There is a growing interest among behavioral ecologists in behavioral syndromes and animal personality. Studies of behavioral syndromes repeatedly measure the same individuals to quantify within-individual consistency and between-individual variation in behavior. Often these studies measure behavior in different contexts or in different behavioral assays to determine whether individual differences in behavior in one context are related to behavior in other contexts, that is, a behavioral syndrome. For studies of behavioral syndromes, there is not universal agreement about whether it is preferable to randomize the order of different assays or to administer them in a fixed order. Here, I articulate the advantages and disadvantages of testing in a randomized or fixed order and offer some recommendations according to the goals and power of the experiment. In general, studies using within-subjects designs that are primarily interested in mean-level differences between treatments should randomize the order that individuals experience different treatments. Under certain conditions, studies of behavioral syndromes should also administer the assays in a randomized order, but only if the study is sufficiently powerful to statistically account for carryover and period effects. If the experimenter is interested in behavioral syndromes that are caused by carryovers, it is often preferable to test in a fixed order. If the experimenter wants to guard against carryovers, but the experiment is not sufficiently powerful to account for carryover and period effects, then a compromise is to test in a fixed order, but to test individuals in the context that is most likely to affect subsequent behavior last.