1. The ability to modify a scheme de nition in one level without a ecting a scheme de nition in a higher level is called data independence.

2. There are two kinds:

Physical data independence

• The ability to modify the physical scheme without causing application programs to be rewritten
• Modi cations at this level are usually to improve performance

Logical data independence

• The ability to modify the conceptual scheme without causing application programs to be rewritten
• Usually done when logical structure of database is altered

3. Logical data independence is harder to achieve as the application programs are usually heavily dependent on the logical structure of the data. An analogy is made to abstract data types in programming languages.