Dataare hierarchically structured, or can be said to have a multilevel structure,when observations can be grouped because they belong to different samplingunits, for example, observations about pupils can be grouped according to theirdifferent classes or schools. Where dataare hierarchically structured, it is conventional to refer to lower-level units(e.g., pupils) and higher-level units (e.g., classes) where the lower-levelunits are nested or grouped within the higher-level units.
See Multilevel modeling (MLM