Managing Information Technology

332 Part III • Acquiring Information Systems

Three system characteristics that are especially
important for analyzing and designing information systems
are the following: determining the system boundary,
breaking down a system into modules (decomposition), and
designing interfaces between old and new systems.

SYSTEM BOUNDARY The system boundarydelineates
what is inside and what is outside a system. A boundary
segregates the environment from the system or delineates
subsystems from each other. A boundary in the systems
world is often arbitrary. That is, we can often choose to
include or exclude any component in the system. The
choice of where to draw the boundary depends on factors
such as these:

1. What can be controlledRecognizing you can’t
   control everything...Elements outside the control of
   the project team are part of the environment, and the
   environment often places a constraint on the system
   scope. For example, if a preexisting billing system is
   treated as part of the environment of a new product
   management system, the product management system
   would be limited to devising products that can be
   priced and billed in ways already supported.


2. What scope is manageable within a given time
   periodMake progress and move on to the next
   job...Complex systems often take so long to
   design and develop that the envisioned systems

solution could no longer be the best choice by the

time the project is complete.

3. The impact of a boundary changeWhile you were
   gone over the weekend, we decided to...As the
   business changes or new information about the organ-
   ization is uncovered, a different system boundary can
   appear to be beneficial. This decision requires careful
   analysis of the impact of such a change.

COMPONENT DECOMPOSITION A system, like an

assembled product, is a set of interrelated components. A

component of a system that is itself viewed as a system (or

a set of interrelated components) is called a subsystem

(module). The components of a subsystem can be further

broken down into more subsystems. The process of

breaking down a system into successive levels of

subsystems, each of which shows more detail, is called

hierarchical (or functional) decomposition. An example is

provided in Figure 8.4. Figure 8.4 (A) shows a high-level

view of the system with two subsystems. Figure 8.4 (B)

shows details about one of these subsystems, Produce

Sales Summary. One important relationship between the

two views of the Produce Sales Summary subsystem is that

there are two inputs to each view and the consolidated

output in (A) matches with the detailed outputs in (B).

Five important goals of hierarchical decomposition

of a system are the following:

1. To cope with the complexity of a system
   Decomposition of a complex system allows us to
   break the system down into understandable pieces.


2. To analyze or change only part of the system
   Decomposition results in specific components at just
   the right level of detail for the job.


3. To design and build each subsystem at different
   timesDecomposition allows us to respond to new
   business needs as resources permit while keeping
   unaffected components intact.


4. To direct the attention of a target audience
   Decomposition allows us to focus on a subset of
   components of importance to a subset of the total
   user population.


5. To allow system components to operate more
   independently Decomposition allows problem
   components to be isolated and components to be
   changed, moved, or replaced with minimal impact
   on other components.

INTERFACES An interface is the point of contact

between a system and its environment or between two

subsystems. In an information system, the functions of an

interface are generally as follows:

Inputs

Outputs

Components

Interfaces

Storage

System

Time cards

Vouchers

Paychecks

W-2 forms

Calculate total

pay

Subtract deduc-

tions

Match time cards

to employees

Sort paychecks by

department

Employee benefits

Pay rates

Payroll

Customer orders

Customer returns

of goods

Monthly sales by

product

Monthly sales by

territory

Accumulate sales

by product and

compare to

forecast

Translate cus-

tomer zip code

into territory

code

Product list

Sales history

Sales forecasts

Sales Tracking

FIGURE 8.3 System Component Examples