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the general concept
Collection of data
[[File:Iris dataset scatterplot. svg|thumb|right|Various plots of the multivariate data set Iris flower data set introduced by Ronald Fisher (1936). In the open data discipline, data set is the unit to measure the information released in a public open data repository. The European data. europa. eu portal aggregates more than a million data sets.

Several characteristics define a data set's structure and properties. These include the number and types of the attributes or variables, and various statistical measures applicable to them, such as standard deviation and kurtosis. The values may be numbers, such as real numbers or integers, for example representing a person's height in centimeters, but may also be nominal data (i. e., not consisting of numerical values), for example representing a person's ethnicity. More generally, values may be of any of the kinds described as a level of measurement. For each variable, the values are normally all of the same kind. Missing values may exist, which must be indicated somehow. In statistics, data sets usually come from actual observations obtained by sampling a statistical population, and each row corresponds to the observations on one element of that population. Data sets may further be generated by algorithms for the purpose of testing certain kinds of software. Some modern statistical analysis software such as SPSS still present their data in the classical data set fashion. If data is missing or suspicious an imputation method may be used to complete a data set.