One of the keys to any statistical inference is that the data involved be obtained
via some formal mechanism that enables the experimenter to make valid judgements on
the question(s) of interest. One of the most common mechanisms for obtaining such
data is that of a simple random sample. Other more structured sampling designs, such
as stratified sampling or probability sampling, are also available to help make sure that
the obtained data collection provides a good representation of the population of
interest. Any such additional structure of this type revolves around how the sample
data themselves should be collected in order to provide an informative image of the
larger population. With any of these approaches, once the sample items have been
chosen the desired measurement(s) is collected from each of the selected items.