structured analysis infsy 570 dr. r. ocker. what is structured analysis? n it focuses on specifying...
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Structured Analysis
Infsy 570
Dr. R. Ocker
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What is structured analysis?
It focuses on specifying what the system or application is required to do.
Elements of structured analysis . Graphical description . Data Flow Diagrams . Data Dictionary: definitions of
elements in the system
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What is structured Design?
Focus on the development of software specifications
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Three primary modules in SDLC
(1) Analysis
(2) Design
(3) Implementation
various phases within each module
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Systems Analysis
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A. Objectives of System Analysis
determine if current system is in trouble
determine appropriate alternatives
make recommendation
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To establish in detail what the system is to do objectives costs benefits implementation process organizational changes required defines who the USERS are Their input and output
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Phase 1 Identify problems, opportunities, and objectives analyst looks at what is occurring in the
business look for problems and opportunities People involved: users, analysts, systems managers
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Phase 1 Activities include: interviewing user management summarizing knowledge obtained estimating scope of project documenting results Output of this phase: feasibility study (report) containing a
problem definition and summarizing the objectives
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Feasibility Study: The Basic Tasks Problem Orientation . Define the problem . Establish system objectives . Identify the users . Establish functional scope
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Feasibility Study
Alternative Specification– Propose options– Cost-Benefit analysis– Assess project risk– Recommend
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Feasibility Study
Technical Do we have the capability to develop the
system? Does the necessary technology exist? Does the proposed system have the
right:– response time, interface,
Can the system be expanded?
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Feasibility Study
Economic Is there an economic payoff? cost of hardware/software/ other benefits in terms of reduced costs opportunity costs
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Phase 2 Determining RequirementsPhase 3 Analyzing System Needs
goal - determine the requirements of the new system -- must obtain a consensus from the user community on the ideal information system
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Requirements
Analyst must understand:
a. the CURRENT SYSTEM
b. what information users need to perform their jobs
c. why and how current system is no longer effective
analyst derives new system requirements from an analysis and synthesis of a,b,& c
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Requirements
People involved in this phase:
analysts, users - operations managers and workers
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Major steps in determining requirements for new system
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1. Requirements capture and analysis The process of deriving system
requirements accomplished through observation of
existing systems, discussions with potential users and task analysis.
very time consuming step
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Analyst needs to know details of current system functions who - the people who are involved what - the business activity where - the environment in which the
work occurs when - the timing of the activity how - how the current procedures are
performed
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2. Requirements definition
document containing an abstract description of:
- user functions the new system is expected to provide
- constraints under which the system must operate.
only specifies the external behavior of the system - does not cover any implementation considerations.
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2. Requirements definition
written without using any specialized notations so that it is understandable by non-systems professionals (e.g., potential users, system procurers).
becomes the focus for much of the work involved in developing system requirements.
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3. Requirements specification
document that details the functions that the system is to include - sometimes called a functional specification
written using a formal systems specification language
more precise than the requirements definition - for use by the technical staff
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3. Requirements specification
creation of this document might be carried out in parallel with some high-level design (this may be essential) as the design and requirements activities influence each other as they develop.
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Tools used to determine requirements sampling and investigating hard data,
interviewing, questionnaires, observation, and prototyping
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CASE tools
DFD - data flow diagram - used to chart the input, processes, and output of the business's functions in a structured graphical form
data dictionary - developed from the DFDs; lists all of the data items used in the system along with their specifications
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Analysts use CASE tools to
1. Increase productivity - draw and modify diagrams easily
2. Improve analyst-user communication - same - draw and modify diagrams easily
CASE tools integrate activities within the SDLC
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types of CASE
1. Upper CASE - used by analysts to create and modify system design
2. Lower CASE - used to generate computer source code
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Using Data Flow Diagrams
structured approach - take a top-down approach to system development
system is defined first at a general level - overview successive refinement occurs until the bottom
(primitive) levels are defined. primitive level - point where specifications can be
translated into lines of code So...system is decomposed into small modules that
perform simple tasks
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Structured Development
the systematic and integrated use of tools and techniques to aid the analysis and design of information systems
structured methodologies use one or more tools to define information flow and processes.
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Structured Development
definition is from top to bottom in increasing levels of detail.
1.major flows and processes identified
2.these are exploded into subprocesses
3.subprocesses are exploded into more detail. this process can continue to the primitive
level, where programming begins directly from the exploded diagram
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Structured terms
data elements - lowest level of information on which a process can act
i.e. DB attributes/record fields - e.g. unit price data stores - places where data are stored; e.g.
files; microfiche, filing cabinets data flows - represent movement of data in a
system; consist of data input and data output e.g. forms, reports, invoices, letters show movement of data about a physical “thing”
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Structured terms
process specifications - definitions of how processes work
data dictionary - document containing definitions of all system data; includes data elements, data structures, data stores, data flows, and process specifications
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Tools
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I. Hierarchy Chart
graphic tool to represent all the tasks or processes in a system
groups them into hierarchical levels one-to-many relationship between upper
and lower levels of the chart node has single parent and >=0 children
nodes all sibling nodes have same level of detail
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I. Hierarchy Chart
functional primitives - bottommost nodes; get translated into programming code
control modules - any node above the functional primitives
topmost node - always single node, ties whole system together
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II. Data Flow Diagrams
DFDs - graphic representation of the flow of data through a system;
can be physical DFDs or logical DFDs
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Logical DFDs shows sources and sinks (destinations) of data identifies and names the logical functions (processes)
of the system identifies and names the groups of data elements that
connect one process to another identifies the data stores each function broken down into more detailed DFD
(levels) descriptions of processes, flows, stores, elements
recorded in data dictionary
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Logical DFDs
All of the above documentation comprises a logical functional specification for an existing or new system. a detailed statement of what the system
does/is to do free from physical considerations of how it
will be implemented
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DFD components (Gane & Sarson Methodology) contains combinations of only four
symbols
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1. External entities
also called sources or sinks people or groups of people who interact with the
current system but are not internal to the system; outside of boundary of our system square is symbol for an external entity to identity an external entity - place a unique,
lower-case alphabetic character in upper-left-hand corner of square. Then give entity a single, descriptive noun as a name.
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2. Processes processes=functions actions performed on data flowing through the
system rounded rectangle - symbol for process each process is identified by a number
corresponding to the level of the process on the hierarchy chart.
each process is named with a simple verb-object pair, i.e. enter transaction
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3. Data Stores
repository for data can be as simple as an in/out box or as
complex as a database open-ended rectangle - symbol of a
data store; open end on right side
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3. Data Stores identified by upper-case alphabetic character
and a digit:– each data store within the same system has the
same alphabetic characters with different digits. when a process stores data data flow arrow goes into data store when process accesses data data flow arrow goes into process avoid crossed data flow lines by repeating the
data stores on the DFD as needed
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4. Data flows
represents the movement of data through the system
data often moves through the system as a form or a report
solid line with arrowhead - symbol for a data flow
each data flow must be identified with a descriptive name
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5. Exploding DFDs into levels
diagrams move from general to specific similar to levels in a menu-driven
system; top-level node is analogous to the main menu
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Context-level
highest level in a DFD lays out sources and sinks and a single
process representing the entire system
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Diagram 0. first-level explosion
contains all the options on the main menu
a leveled set of DFDs has a 1-to-1 correspondence with the levels on the hierarchy chart
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Developing DFDs
Diagram 0 - explosion of context diagram may include from 3 to 9 processes ignore exception handling processes for
the first 2-3 levels of a DFD each process numbered with an integer includes major data stores and all
external entities
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Creating child diagrams each process on diagram 0 can be exploded
to create a more detailed child diagram parent - process on diagram 0 that is
exploded child - resulting diagram vertical balancing - a child diagram cannot
produce output or receive input that the parent process does not also produce or receive
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Creating child diagrams all data flow in or out of the parent process must
be shown flowing in or out of the child diagram child diagram given the same number as its
parent process in Diagram 0 (e.g., process 3 would explode to diagram 3)
processes on the child diagram are numbered using the parent process number, a decimal point, and a unique number for each child process
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Creating child diagrams external entities - not shown on child
diagrams below Diagram 0 child diagram may include data stores not
shown on the parent process processes may or may not be exploded,
depending on level of complexity primitive process - when a process is shown
at its lowest level of detail, cannot be exploded any further
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Some general rules
1.a DFD level should contain from 3-9 processes
2.data flows should not split into two or more different flows
3.all data flows must EITHER originate or terminate at a process
4.processes need to have at least one input data flow and one output data flow
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Some general rules
5. each child diagram should have the same input and output data flow as the parent process
6. avoid crossing data flow lines by repeating data stores and external entities
7. do not show exception handling processes until the lower levels of the DFD
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Physical DFDs vs. Logical DFDs
physical DFDs - contain both what processes are in the system and how the processes operate; show movement of things
allow us to understand how the current system works
can be restrictive in the design process
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logical DFDs
contains only the minimal data that flow through the system, independent of any devices, persons, forms, or specific physical implementations;
implementation-free