Difference Between Top Down and Bottom Up Approach

Sr. No.	Top-Down Approach	Bottom-Up Approach
1	In this approach an overview of the system is first formulated, specifying but not detailing any first-level subsystems. Each Subsystem is then refined in yet greater detail,sometimes in many additional subsystem levels, until the entire specification is reduced to base elements.	In this approach the individual base elements of the system are first Specified in great detail. These elements are then linked together to form larger subsystems, which then in turn are linked, sometimes in many levels, until a complete top-level System is formed.
2	This Approach is used in the development of brand new systems.	This approach is used in existing Systems.
3	Top down design begins the design with the main or top-level module, and progresses downward to the lowest level modules or subsystems.	Bottom up design begins the design with the lowest level modules or subsystems, and Progresses upward to the main program, module, or subsystem.
4	It is suitable only if the specifications are clearly known.	It is suitable if a system is to be built from an existing system.
5	It is suitable if a pyramid type of process model is being used.	It is suitable if an iterative enhancement type of process is being followed.