Surveys
Some types of data are obtained from experiments. In experiments, we actively change some characteristics of each individual — choosing an experimental treatment.
Other types of data are obtained by selecting a sample of individuals in some way and simply recording information about them — a survey.
Summarising data
Survey data sets are often large and are summarised by numerical
values called summary
statistics.
Natural variability between individuals means that summary statistics must be considered as random quantities — similarly collected data would result in different values.
A important role of statistics is to understand and describe the randomness of such summary statistics.