Classification of research based on the scope of the population covered
Statistical surveys can be divided, depending on the number of units under observation covered by the survey, into censuses and partial surveys.
Census: it covers the entire general population, i.e. each unit of the population is covered by the survey.
Partial survey: it covers certain selected units of the population (sample), i.e. only a part of the general population is observed; partial statistical surveys of official statistics include surveys with random sample selection (representative surveys), in which each sampling unit has a certain probability of being included in the sample, and non-randomselection – purposive sampling.
The basic census conducted by official statistics is the population and housing census. It provides information necessary to determine the state and structure of the phenomenon at a precisely defined moment; it is a special survey carried out usually every 10 years, based on collecting information on appropriately prepared census forms.
A representative survey means that in order to examine the characteristics of the entire population, only a certain group of units is selected for the survey – a random sample from the general population; the probability theory enables to determine the magnitude of error made when generalizing the results from a random sample to the entire population.