  Snowball designs are the most natural of the network sampling designs. They
have many desirable properties for sampling hidden and hard-to reach
populations. They have been under-used in recent years because simple
design-based estimators and confidence intervals have not been available for
them. The needed estimation methods are supplied in this paper. Snowball
sampling methods and accurate estimators with them are needed for sampling of
the people exposed to the animals from which new coronavirus outbreaks
originate, and to sample the animal populations to which they are exposed.
Accurate estimates are needed to evaluate the effectiveness of interventions to
reduce the risk to the people exposed to the animals. In this way the
frequencies of major outbreaks and pandemics can be reduced. Snowball designs
are needed in studies of sexual and opioid networks through which HIV can
spread explosively, so that prevention intervention methods can be developed,
accurately assessed, and effectively distributed.
