Suppose we analyze as level 1 variable �tooth health status�.

�Tooth health status� is an aggregation of several tooth characteristics (this variable follows a normal distribution, it has a mean and standard deviation).

There is a structure in this data, a hierarchy. We have TEETH as level 1 and MOUTHS as level 2.

We also create �arbitrarily defined mouths� (red circles). These mouths are only a mix of teeth without any natural or anatomical criterion.