Coding "missing values" - standards, customs, tricks of the trade?

Do any standards exist for the representation of "missing" values? Failing that, any widely used customs? Or tricks of the trade? Ten years ago, while working with medical and scientific data for which partial results were computerized pending final reports from a laboratory, I was accustomed to flagging such "missing, but expected" values with a project-specific code; this distinguished "missing" cases from values that were already firmly known to be zero, null, void, "not applicable", lost, unobtainable, or whatever. At that time I took a quick look and could not find evidence of any standards by which to design for this aspect of data management. Having recently become re-involved with some datasets sharing this characteristic of having an unpredictable arrival date for some values, I wonder what standards or approaches may be current practice. Pointers to textbooks, journal articles etc. would be welcome. Thanks, Steve Fick Inso Corporation SteveFick@inso.com or infosrfp@world.std.com