Re: RM formalism supporting partial information
Date: Wed, 28 Nov 2007 13:24:26 GMT
Jan Hidders wrote:
> On 28 nov, 01:58, David BL <davi..._at_iinet.net.au> wrote:
>> Consider a query to find all the 27 year old pilots from a census >> recorded in an RDB. If the age or occupation is missing we could >> think of the person as a possible answer.
> I believe there is a terminology problem here concerning the terms
> "possible answers" and "certain answers". In the context of research
> on incomplete databases (i.e. anywhere the classical CWA does not
> apply fully) that usually means the following. Given a query and the
> assumptions about "closedness" the set all tuples with the right
> header can be partitioned into three groups: the certain answers
> (those that are certain to be in the result of the query on the
> omniscient database), the possible answers (those that might be in the
> aforementioned result) and the impossible answers (those that are
> certain not to be in the aforementioned result).
> In that sense the tuple describing the person you mentioned above
> (presuming it is projected on the non-null fields) is a certain
> answer, not a possible answer.
When it comes to a public census I believe the possible answers or non-answers are planned for. As Bob B pointed out a "Don't Know" response or even a refusal is often considered a specific answer, ie., some number of those is expected. Interesting that even statisticians who are more interested in probability than db theory do this. Seems quite different from the usual null examples. (I'm not touting census methods in general - I've seen outrageous cheating by census-takers, making up answers or even non-existent people in order to meet quota maximums for DK's/NA's.) Received on Wed Nov 28 2007 - 14:24:26 CET