IValueDefinition is a bit confusing. See http://en.wikipedia.org/wiki/Measurement for reference. Measurement is a process of measuring things which can be measured, this is true for both quantitative and qualitative types of data. In other words are IMeasurable. How they can be measured is defined by IQuantity and IQuality interfaces which both defining something which is IMeasurable.
/* any type of measurements, both quality and quantity are measurements */
interface IMeasurable
{
...
}
interface IQuantity : IMeasurable
{
...
}
interface IQuality : IMeasurable
{
...
}
