  In spite of recent contributions to the literature, informative cluster size
settings are not well known and understood. In this paper, we give a formal
definition of the problem and describe it from different viewpoints. Data
generating mechanisms, parametric and nonparametric models are considered in
light of examples. Our emphasis is on nonparametric and robust approaches to
the inference on the marginal distribution. Descriptive statistics and
parameters of interest are defined as functionals and they are accompanied with
a generally applicable testing procedure. The theory is illustrated with an
example on patients with incomplete spinal cord injuries.
