With quantities, we can easily deal with multiple currencies, rather than being tied to one currency. We saw that units have been used in modeling Quantity. Another concept that is interesting is, units allow us to convert quantities from one unit to another. Conversion between inches and feet can be achieved by defining a conversion ratio from feet to inches with the number 12. As is evident, this model can handle most but not all kinds of conversion.
Some conversions require more than a simple multiplication.
For monetary values, whose units are currencies, the conversion ratios are not constant over time. This problem can be dealt with by having an attribute that indicates the time of applicability of the ratio. An observation is, making a qualitative statement about a person or a situation. Observations are similar to measurements where we make quantitative statements about an object. Consider measuring a person's gender.