Information Systems Development MIS 331 Logical and Logic

Information Systems Development MIS 331 Logical and Logic Modeling 12/11/2021 MIS 331 1

Agenda • System Modeling – Logical versus Physical – Process Modeling with Data Flow Diagrams – Logic Modeling 12/11/2021 MIS 331 2

Process Modeling • Graphic representation of functions or processes which capture, manipulate, store, and distribute data. – System boundary is defined by where the data enters and exits the system • Takes advantage of the concept of functional decomposition – Process models can be constructed in a layered fashion such that increasing levels of complexity can be illustrated 12/11/2021 MIS 331 3

Data Flow Diagram (DFD) Gane & Sarson Nomenclature Identifier Process Name File Name Entity Name Process Data Store Source/Sink Data Flow 12/11/2021 MIS 331 4

Process versus Entity • How do we decide if something is a process or an entity? – If this “something” alters data as a function of the system then it is a process. • creates a report, sorts, calculates, reads or writes, etc. – If this “something” simply generates data as input to the system or accepts data as output from the system then it is an entity. • customer submits an order • customer receives item ordered • Processes are inside the system boundary, entities are outside 12/11/2021 MIS 331 5

Coding the Process Identifier • Context level – defines the entire system and its external entities • Level 0 – defines the primary individual processes • identifier is X. 0 where X equals the process number • Level 1 – first decomposition of Level 0 processes into sub-processes • identifier is X. Y. 0 where Y equals sub-process number • Level n – the result of n nested decompositions • identifier is X. Y. . . n. 0 • Functional Primitive Level – lowest logical level where no further decomposition yields benefits 12/11/2021 MIS 331 6

Simple System Decomposition 12/11/2021 MIS 331 7

The Rules of DFDs In a nutshell… All data flows must begin and/or end at a process, because data flows either initiate or result from a process. 12/11/2021 MIS 331 8

Three Basic Illegal Data Flows Identifier Entity Name Process Name Black Hole File Name Miracle Identifier Process Name 1. 0 Employee File 12/11/2021 Create Paychecks Employee MIS 331 Gray Hole 9

Logic Modeling • Logic Modeling is logical but it is not Logical Modeling – Logical Modeling is the representation of processes and their relationships to other processes. – Logic Modeling is the representation of the internal structure and functionality of processes and subprocesses. • How are inputs converted to output? • When does a process occur? • When should a process not occur? 12/11/2021 MIS 331 10

Logic Modeling Tools • Structured English – just a modified version of good ol’ English • Decision tables and trees – models of conditions and actions as tables or graphs • State-transition diagrams and tables – shows the various states a system component can assume – events that result in a transition from one state to another 12/11/2021 MIS 331 11

Structured English • Relies on strong, imperative verbs and noun phrases – read, write, sort, move, add, begin, end, etc. – uses only nouns and terms previously defined in the data dictionary • Undefined adjectives or adverbs are not used unless clearly defined in the data dictionary as value ranges for data element descriptions • avoids the construction of compound sentences Take the hours_ wrkd and multiply them by Calculate gross_pay using the formula: the pay_rate then put this value into the hours_ wrkdand * pay_rate. field called gross_pay. 12/11/2021 MIS 331 12

Decision Tables • Better than Structured English when logic becomes nested and complex If Employee=“S” then pay base salary else, If Employee=“H” then calculate wage and If hours_wrkd < 40 then produce absence report else, If hours_wrkd are > 40 then calculate overtime. 12/11/2021 MIS 331 13

Decision Tree • Depicts the decision scenarios as a connected series of nodes and branches which ultimately end in one or more actions – probabilistic and nonprobabilistic • probabilistic trees allow for condition probabilities to be carried throughout the decision process – Nodes are decision points – Branches are alternative paths – Actions are the expected result of a decision based on the variables or constraints in place 12/11/2021 MIS 331 14

Decision Tree If Employee=“S” then pay base salary else, Salaried? Pay base salary 1 If Employee=“H” then calculate wage and If hours_wrkd < 40 then produce absence report else, If hours_wrkd are > 40 then calculate overtime. < 40 Pay hourly wage Absence report = 40 Hourly? 2 Pay hourly wage > 40 Pay hourly wage Pay overtime wage 12/11/2021 MIS 331 15

When to Use What 12/11/2021 MIS 331 16

For Next Time • More on DFDs on Thursday • Get a good grip on this material – It may show up on a knowledge demonstration • Case #1 Presentation on Tuesday – Get prepared for Group A’s presentation – Know Case #1 inside and out. • Start thinking about KD #1 – It will be here before you know it… 12/11/2021 MIS 331 17