In the context of data migration and system replacement, what is meant by the term legacy data?

Introduction / Context:
When organizations replace old information systems with new ones, they must decide what to do with the existing data. The term legacy data is widely used in database design, data warehousing, and system migration projects. Understanding this term is important when planning conversions, interfaces, and data cleaning activities. This question asks for the correct definition of legacy data in the typical context of moving from an old system to a new one.

Given Data / Assumptions:

There is an existing information system that has been in use for some period of time.
A new system is being installed to replace or supplement the old one.
The old system contains data that may need to be moved, cleaned, or integrated.
The term legacy data refers to data in relation to these old and new systems.
We are not restricting the term to any particular storage technology or file format.

Concept / Approach:
Legacy systems are systems that were used before a new system was introduced and that may still be running for backward compatibility or phased out over time. Legacy data therefore refers to data that resides in these older systems. This data is often stored in formats that may not match modern standards, and it may contain inconsistencies accumulated over years of use. When planning data migration or integration, designers must treat legacy data as a source to be cleansed, transformed, and loaded into the new environment. The key idea is that legacy data belongs to the pre existing system, not the newly installed one.

Step-by-Step Solution:
Step 1: Identify the relationship between the terms legacy system and new system. A legacy system is the system that existed before and is often considered outdated or in need of replacement. Step 2: Recognize that legacy data is simply the data that is stored within that older system, regardless of whether it is in files, databases, or other storage structures. Step 3: Compare this understanding with the answer options. The option that states that legacy data is data contained by a system used prior to the installation of a new system matches this interpretation exactly. Step 4: Confirm that the other options either refer to new system data, rejected data, or generic file system data without the historical context, and therefore do not fit the standard definition.

Verification / Alternative check:
Imagine an organization that used an old mainframe based system for payroll and then moves to a modern relational database based system. The data that existed in the mainframe files before migration, including employee records and pay history, is legacy data. Once that data is moved and cleaned into the new database, it becomes part of the new system and is no longer called legacy data in that context. This simple scenario confirms that legacy data is tied to the old system that precedes the new installation.

Why Other Options Are Wrong:
Data stored in a newly installed system is not legacy data, because legacy implies something older that existed before the current system.
Data that is rejected during installation is often called invalid, corrupt, or rejected data, not legacy data. Legacy data is still real data, even if it requires cleaning.
Data stored in a file system without reference to its age or system context is just file system data. It becomes legacy data only in relation to the introduction of a new system that supersedes the old one.

Common Pitfalls:
One pitfall is to treat legacy data as inherently bad or useless. In reality, legacy data can be very valuable, but it often needs cleaning and transformation. Another mistake is to underestimate the effort required to migrate legacy data into a new schema, especially when there are differences in data types, codes, and business rules. Designers should also be careful not to assume that all legacy data must be moved at once; sometimes only a subset of historical data is needed. Clear understanding of the term legacy data helps in planning realistic migration strategies.

Final Answer:
Legacy data is best described as data held by a system that was used before the installation of a new system.