APA Dictionary of Psychology

the situation in which the absence of certain points from a data set is unrelated to the nature of the particular items but may be explained by other variables. For example, assume a researcher surveyed 100 students on a college campus about their weekly amount of alcohol, drug, and cigarette use but 12 individuals failed to note their use for an item. If all 12 people are male, and if there is no consistency as to which item was left blank by which person, then the pattern of responses on the survey could be related to sex (perhaps the men on campus are less likely to answer any type of evaluative item than are the women). If so, the values would be missing at random. When data are missing at random there is a danger of biasing study results.