Characteristically, a problem of contemporary social science is obtaining from the members of a sample of n subjects information concerning k variables that are of interest to the investigator. The usual methods of solving this problem are the use of questionnaires, interviews, and/or observation schedules. An immense amount of literature is available on these data collection instruments, which need not be reviewed here. But the instruments rarely succeed entirely in solving the problem; all are subject to the further difficulty that for some fraction of the sample and some variables the required information proves to be unobtainable.