## 2 Materials and Methods
The behaviour illustrated was counted within each observation period for each pig and each behaviour (initiating tail-biting behaviour, receiving tail-biting behaviour, initiating ear-biting behaviour and receiving ear-biting behaviour) separately. To simplify the comparison between different lengths of observation periods, the amount of behaviour \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Observation Period** & & & & & \\
**(OP)** & **Sampling Method** & **Hours Covered** & **Hours Analysed** & **Number of Different OP** & **Limited to** \\  & & & & & & **Activity Time** \\ \hline
12 h & continuous sampling & 12 & 12 & 1 & no \\
6 h & continuous sampling & 6 & 6 & 2 & no \\
3 h & continuous sampling & 3 & 3 & 2 & yes \\
1 h & continuous sampling & 1 & 1 & 6 & yes \\
0.5 h & continuous sampling & 0.5 & 0.5 & 12 & yes \\
24 \(\times\) 10 min & time sampling & 12 & 4 & 3 & no \\
12 \(\times\) 10 min & time sampling & 6 & 2 & 6 & no \\
2 \(\times\) 6 \(\times\) 10 min & time sampling & 6 & 2 & 3 & yes \\
6 \(\times\) 10 min & time sampling & 3 & 1 & 6 & yes \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Overview of the observation periods used (OP). The activity times were 08:30 to 11:30 and 14:30 to 17:30. Figure 1: Overview of observation periods used (OP). The grey sections display the earliest examples of each kind of observation period per day. The activity times were 08:30 to 11:30 and 14:30 to 17:30.  
illustrated was adjusted to the amount of shown behaviour per hour. Then, all observation periods were compared twice for each behaviour separately: Comparison 1; compared all observation periods to the continuous observation period of 12 h to analyse the agreement to the reference, i.e., the representation of reality. Comparison 2; compared all observation periods of the same kind (continuous sampling or time sampling) and length with each other to analyse the agreement between the observation periods. It was known that the amount of biting behaviour shown increased over the four observation days [19]. Thus, differences between the days would arise from the developing tail-biting outbreak and not from measurement errors of the chosen sampling method or observation period. Because this study focused on the differences of observation periods and sampling methods, each day was used as an independent unit and all comparisons were limited to the observation periods within a day. Different measurements of accuracy were used for the comparisons. On the one hand, a measurement relied on the numerical agreement. The amount of behaviour of a pig shown in one observation period was compared to the amount of behaviour of the same pig shown in the other observation period. However, a tolerance range was applied, because it was highly unlikely to observe an animal showing behaviour in two observation periods for the exact same amount of time. The robustness of the database of this study was analysed beforehand for the effect of missing data. It could be shown that observation periods of different length were robust against missing data if an error rate of 10% was not exceeded. Based on these results, the tolerance range was set to \(\pm\)10% as values within this tolerance range could still be used for an agreement. Other tolerance ranges were also tested. Wider tolerance ranges increased the resulting accuracy scores and their variance; however, the ratio between the mean accuracy scores of the different observation periods did not change. Still, the tolerance range was set to \(\pm\)10%. Therefore, the agreement was either 1 (the amount of behaviour of a pig shown in one observation period was within the range of the amount of behaviour \(\pm\)10% shown in the second observation period) or 0 (the amount of behaviour of a pig was not within the range of the amount of behaviour \(\pm\)10% shown in the second observation period). The percentage of agreements (PA) was then calculated per pen. On the other hand, measurements relied on the stability of the ranking order within a pen.