What is a logical flow diagram? Definition, uses and example

By Indeed Editorial Team

Published 8 April 2022

The Indeed Editorial Team comprises a diverse and talented team of writers, researchers and subject matter experts equipped with Indeed's data and insights to deliver useful tips to help guide your career journey.

Many companies use data flow diagrams (DFDs) to map and improve their business processes. Data flow diagrams generally fall into one of two categories, logical or physical data flow diagrams, where each has different functions. If you work in a field that uses DFDs, such as IT, system analysis or business analysis, it can be useful to understand what a logical flow diagram is and how it compares to a physical flow diagram. In this article, we define the two terms, explain the benefits of each and provide a helpful example.

What is a logical flow diagram vs a physical flow diagram?

The answer to 'What is a logical flow diagram?' is essentially a way to map a company's business activities. It shows the flow of information needed for a process to take place. For example, a business might use it to show the process of a customer making an online purchase. This would start with the customer adding items to their basket; then, the system would look up the prices and display a total. The customer would then make a payment and get a receipt. This might then trigger other actions, such as deducting items from an internal stock list.

A physical data flow diagram maps how you implement a system. It details the hardware, software, people, paper files and other resources needed for the system to work. In the example above, the physical flow diagram would include the database that stores the prices and the software that processes the payment and automatically sends a receipt to the customer. While a logical flow diagram 'shows' what happens in a process, a physical data flow diagram provides information on 'how' you achieve this. People often use the two types of DFD together.

Related: A beginner's guide to process mapping (with helpful tips)

The purpose and benefits of logical and physical DFDs

Businesses can use both types of DFD to map and improve their systems and processes. They can start by creating a logical DFD of their current processes, identifying any problem areas or steps that they could remove or simplify. Then, they can create a new logical DFD that details the new process they want to implement. A big benefit of logical DFDs compared to physical ones is that they are more easily understandable by people without a technical background, meaning that the people actually involved in the process can comprehend them and provide feedback.

Once a company has created a logical DFD that shows their new processes, they can use it to figure out the best methods to achieve each step and map these on a physical DFD. For example, if a company knows based on their logical DFD that they need a piece of software to issue payment receipts to customers, they can research the best program for the job. When the physical DFD finishes, it provides an entire implementation plan detailing all of the software, hardware, new hires or other physical resources required to implement the new process.

Related: Guide to workflow: definition, components, processes and uses

Components and symbols used in DFDs

Data flow diagrams have different sections, which mean slightly different things depending on whether you're making a logical or a physical DFD. There are also various symbol systems you can use to represent each element. The two main systems are named after their creators, Yourdon-Coad and Gane-Sarson, and include these components:

External entity

An external entity is a person or thing outside the company, such as a customer or user. The symbol for an external entity in both Yourdon-Coad and Gane-Sarson is a square. You might use this component in a DFD to show an action taken by someone outside the company, such as a customer making a purchase online.

Process

The function of a process in a data flow diagram is slightly different in physical and logical DFDs. In a logical DFD, processes are business activities, such as advertising for an open position, computing the total cost of a customer order or replying to an email. In a physical DFD, a process is a piece of software, a manual procedure or another way of processing data. The symbol for a process in a DFD is a circle in the Yourdon-Code system and a square with a horizontal line through it in Gane-Sarson.

Data store

In a logical data flow diagram, a data store is a collection of information, no matter how or where it's stored. A company's list of product prices is an example of a data store. In a physical data flow diagram, a data store is an actual database, computer file or paper file where you store the information. The symbol for a data store is a rectangle in both symbol systems, with a vertical line through it in Gane-Sarson.

Related: A guide to the different types of charts and graphs

Data flow

The purpose of both types of data flow diagrams is to visually represent the stream of information from one source to another as it relates to business processes. An arrow represents data flow in both symbol systems. In a logical data flow diagram, the data flow simply describes where data flows from and to, such as an employee looking up the price of an item. A physical flow diagram details the actual process used, such as scanning the universal product code (UPC or bar code) printed on the item's packaging.

Different uses for data flow diagrams

Professionals might use data flow diagrams in different ways in various fields and industries. Here are some common uses for DFDs:

Software engineering

Data flow diagrams originated in the software development field, where developers use them for mapping current processes and planning new ones. For example, they might use a logical data flow diagram to map the current activities necessary for a process and a new logical data flow diagram to determine better ways of achieving the same goal. They can then use this diagram to identify the software, code and other resources needed to meet their requirements.

Business analysis

In business analysis, a detailed logical data flow diagram can help to reveal business requirements that might otherwise have gone unnoticed until much later in the process, which could cause delays and even lost income. Since they are easy for non-technical professionals to understand, the people who are personally involved in business operations and activities can provide their feedback and suggestions for improving processes. They can then fill in the logical DFD with the necessary computer programmes, human resources and other requirements for a physical DFD.

Related: How to become a business analyst (with roles and salaries)

Office and administration

Professionals working in office management or administration can use logical DFDs to map the processes and systems necessary for the office to function. They can then model better functionality on a new logical DFD, eliminating redundant steps and identifying areas where they could automate tasks to save time. Using this diagram, they can determine which software or devices might help optimise office processes and map them on a physical DFD.

Healthcare

The healthcare field involves a huge amount of data flow, such as patient information. Healthcare management professionals can use logical DFDs to map current processes and identify shortcomings. This forms the basis for a new logical DFD representing a new, improved process. For example, the process might include adding certain routine tests to patients during intake to eliminate common conditions when performing diagnoses. Users could outline equipment, software or employees on a physical DFD.

Logical data flow example

ABC Software and Services is an IT company whose HR department wants to update its procedures for tracking, interviewing and hiring new employees:

The HR departments create a logical DFD to map their current process to help them spot any areas for improvement and list all business activities that currently occur when sourcing and assessing new candidates. This includes writing and posting job adverts, entering applicants into a company database, alerting hiring managers, tracking process stages and alerting candidates if they were successful.

The DFD helps them lay out the current data flow and provides a basis for them to develop a better system, which they can then map in a new DFD. Based on this new process, hiring managers can receive timely alerts, which allow them to access CVs and applications more easily and automatically alert candidates of rejected applications. Based on this logical data flow diagram, the HR team begins discussing the software, people and other resources that may help them optimise their processes, which they can then lay out in a physical DFD.

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