## 4 Discussion
Besides, for the measurement PA, it has to be considered when the animals are observed and whether the behaviour is regularly shown throughout the day, as there are differences in the social interaction of animals during activity or resting periods [23]. In this study, for example, tail-biting behaviour was shown more often during the activity time in the morning or in the afternoon. Thus, focusing the observations on activity time and applying this to the whole day overestimated certain behaviour, since resting periods were neglected. As a result, the accuracy scores of observation periods focusing on the activity time were worse for the measurement PA compared to observation periods which included inactivity time. Moreover, the comparison between the observation periods of the same kind and length produced mostly low accuracy scores. Therefore, the starting point of the observation can also affect the results considerably, which was demonstrated by [PERSON] et al. [24] as well. This can complicate comparability between studies if animals are observed at different times or if animals differ in their daily routine because of variations in, for example, feeding schedules. Thus, sampling method, observation length as well as observation time should be carefully considered. However, the question is whether it is possible to distinguish between bad, moderate or good accuracy. Or whether it is possible to determine a required minimum analysis time per day, bearing in mind that it is unusual for studies on biting behaviour to observe animals for longer than 2 h per day [13; 14; 15; 16; 17; 18]? There are defined thresholds for the evaluation of correlation coefficients [2]. Applying them to \(\text{R}_{\text{S}}{}^{2}\), an accuracy score between 0.16-0.49 results in moderate accuracy, between 0.49-0.81 in good accuracy and an accuracy score above 0.81 results in very good accuracy. If these thresholds are applied to the other measurements of accuracy as well, no observation period would exceed the threshold for very good accuracy on average. Moreover, not all observation periods of a total analysis time of 2 h or longer would exceed the threshold for good accuracy on average. For the measurement PA, only mean accuracy scores of the observation periods longer than 4 h would produce a moderate accuracy. Thus, the ranking order could be estimated with an at least moderate validity, whereas the amount of behaviour shown could not be estimated with an at least moderate validity except for the longest tested observation periods. Nevertheless, these are results of biting behaviour that is difficult to observe because it is irregular and has a short duration [9]. Thus, it should be analysed whether the results presented are transferable to other types of behaviour as well. As different studies have demonstrated, behaviour can be efficiently and accurately estimated using scan sampling for behaviour with a long duration (e.g., resting) [4; 6; 8; 9; 10; 11] or time sampling for behaviour with a moderate duration (e.g., feeding and locomotion) [3; 8]. However, in other studies on pigs, it was not possible to accurately estimate different behaviour with short duration using scan sampling, such as pain associated behaviour [12] or agonistic or sexual behaviour [9]. Hence, the validity of observations of short-term and irregular behaviour should be investigated in greater detail to approve results of existing studies and to determine optimal observation techniques for future studies.