Is there a measure at all? This is a different question than the value of the measurement. (Though "I don't know" doesn't map very well to rational numbers...

Is there a measure at all? This is a different question than the value of the measurement. (Though "I don't know" doesn't map very well to rational numbers... maybe a float NaN could be abused). That's why many databases allow for (the option of) types to include a NULL value that is handled specially. When you are rolling your own data design it is a legitimate question to ask: "Will an incomplete version of this object exist?" If the answer is yes, then the design should accommodate making that distinction.