Managing Information Technology

346 Part III • Acquiring Information Systems

D1

Purchase

Order

D3AccountVendor

Accounting

D4CheckingAccount

D2 PayableInvoice

D5 Receipts

Vendor

Invoice

Matching

Purchase

Order

Matching

Stock Receipt

Slips

1.0

Approve

Invoice

Approved

Invoice

System

Boundary

Check

Approved

Invoice

New

Account

Balance

4.0

Summarize

AP

Check Register,

G/L Summary

Checking

Register

New

Checking

Balance

Old

Checking

Balance

Account

Balances

Due

Account

Old

Account

Vendor Balance

2.0

Update

Vendor

Account

3.0

Prepare

Vendor

Check

FIGURE 8.13 Top-Level Data Flow Diagram for Accounts Payable System

payable organizational data flows as we want them
to be without implying computerization or any other
form of new system implementation.
In addition to diagrams such as in Figure 8.13, each
external entity, process, data flow, and data store is docu-
mented as to its content. The documentation also shows how
the components are related; for example, the description for
the Vendor entity would include both inbound and outbound
data flows. Similarly, the data store documentation includes
the individual data elements that are input into the store and
matches them to output descriptions.
The accuracy and completeness of a DFD model is
crucial for the process of converting the Logical To-Be model
into the Physical To-Be design. However, prior to commenc-
ing this physical design step, additional logical modeling is re-
quired to define the system’s data elements and relationships.

The most common approach to maintaining the

metadata in a DFD is to create a data dictionary/direc-

tory (DD/D), a concept introduced in Chapter 4. The

goal of the DD/D is to maintain the metadata as

completely as possible; these entries should err on the

side of too much information, rather than too little. This

is also the place to capture whether elements are

calculated, how many decimal places are required, and

how an element may be referred to in external systems

that reference it. Figure 8.14 shows a typical data

dictionary entry for the data element Purchase Order

(PO) Number. Metadata will exist for every process, data

flow, external entity, and data store on DFDs, as well as

the data elements included within each data store.

In addition to the detail at the data element level, the

relationships between entities must be determined. A data