Sampling rules: ad libitum, focal, scan and behaviour sampling

When deciding on systematic rules for recording behaviour, two levels of decision must be made. The first, which we refer to as sampling rules, specifies which subjects to watch and when. This covers the distinction between ad libitum sampling, focal sampling, scan sampling and behaviour sampling. The second, which we refer to as recording rules, specifies how the behaviour is recorded. This covers the distinction between continuous recording and time sampling (which, in turn, is divided into instantaneous sampling and one-zero sampling; see Fig. 6.1). In this section, we consider the four different sampling rules.

1 Ad libitum sampling means that no systematic constraints are placed on what is recorded or when. The observer simply notes down whatever is visible and seems relevant at the time.

Clearly, the problem with this method is that observations will be biased towards those behaviour patterns and individuals which happen to be most conspicuous. For example, in a study of agonistic behaviour in rhesus monkeys, Bernstein (1991) found that ad libitum sampling tended to miss brief responses and under-estimated the involvement of younger individuals in social interactions. Provided this important limitation is borne in mind, however, ad libitum sampling can be useful during preliminary observations, or for recording rare but important events.

2 Focal sampling (or ‘focal animal sampling’) means observing one individual (or one dyad, one litter or some other unit) for a specified amount of time and recording all instances of its behaviour – usually for several different categories of behaviour.