2.1 Rule 1: a site must be sampled during at least 3 winters

The models take both a site and a winter effect into account. When a site is sampled during only one or two winters, then model has a hard time separating the site and winter effect. This can lead to an unstable model. Therefore it is safer to restrict the data to sites with a sufficient number of visits over several winters.

Table 2.1 displays how many sites have data from 1, 2, 3, … different winters. 37 sites have data from less than 3 different winters and are currently ignored.

Table 2.1: Number of sites with data from a number of winters.
winters sites
1 13
2 24
3 14
4 10
5 12