A group was formed as part of caBIG (Cancer Bioinformatics Grid, affiliated with the NIH) to investigate cataloging a list of reasons why information would be missing from biomedical research data sets.
This is where I originally discovered the “flavors of NULL” discussion within the HL7 standard – it was an influencing approach for the caBIG MVR group.
The group does have a collaboration page, but it’s empty. It’s unclear whether these guys are still working on this. The whitepaper has a good Q&A section that deals with their approach.