Failure to obtain all the intended measurements or responses from all the selected sample members is called nonresponse. Nonresponse causes missing data, i.e. results in a data set whose size for the study variable y is smaller than planned. Two types of missing data (unit nonresponse and item nonresponse) are distinguished for a sample element. See pages 112-113 in Lehtonen, R & Pahkinen, E. (2004)

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