Missing data

Instances of values, for one or more variables, which are not included within a dataset and can be identified as missing by the absence of values or by a designated code to indicate that data values are missing. Missing research data values are assigned when the information being collected is missing, which can happen for several reasons – among them, such as failure of an instrument or, within survey research, the respondent refused to answer or the question was inapplicable for that particular respondent because of previous responses.