Document comparison, also known as redlining, is a computer process by which changes are identified between two versions of the same document for the purposes of document editing and review. Document comparison is a common task in the legal and financial industries.

The software-based document comparison process compares a reference document to a target document, and produces a third document which indicates (by colored highlighting or by differing font characteristics) information (text, graphics, formulas, etc.) that has either been added to or removed from the reference document to produce the target document.

Common documents formats for comparison include word processing

Word processing

Word processing is the creation of documents using a word processor. It can also refer to advanced shorthand techniques, sometimes used in specialized contexts with a specially modified typewriter.-External links:...

 documents (e.g. Microsoft Word

Microsoft Word

Microsoft Word is a word processor designed by Microsoft. It was first released in 1983 under the name Multi-Tool Word for Xenix systems. Subsequent versions were later written for several other platforms including IBM PCs running DOS , the Apple Macintosh , the AT&T Unix PC , Atari ST , SCO UNIX,...

), spreadsheet

Spreadsheet

A spreadsheet is a computer application that simulates a paper accounting worksheet. It displays multiple cells usually in a two-dimensional matrix or grid consisting of rows and columns. Each cell contains alphanumeric text, numeric values or formulas...

s, presentations (e.g. PowerPoint

Microsoft PowerPoint

Microsoft PowerPoint, usually just called PowerPoint, is a non-free commercial presentation program developed by Microsoft. It is part of the Microsoft Office suite, and runs on Microsoft Windows and Apple's Mac OS X operating system...

), and Portable Document Format (PDF) documents.

Overview

In the broadest definition, document comparison can refer to any act of marking changes made between two versions of the same document and presenting those changes in a third document via a graphic user interface (GUI). There are several variants in the types of changes registered through the process of document comparison. Some programs limit comparison to solely text and table content in word processing documents, while others register changes made in spreadsheets and presentations , along with changes made in versions of PDF documents. Certain programs also exist that compare changes made to objects like JPEG

JPEG

In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....

, TIFF, BMP and PNG images embedded in documents.
Document comparison solutions redline changes made to the following types of documents:

Word processing documents	Text in paragraphs and in text boxes; bullets and numbering; tables of content; applied styles; design and layout elements; tables, including additions and deletions of rows; embedded object Object Linking and Embedding Object Linking and Embedding is a technology developed by Microsoft that allows embedding and linking to documents and other objects. For developers, it brought OLE Control eXtension , a way to develop and use custom user interface elements... s ; inserted images.
Spreadsheets	Values; formulas; additions and deletions to rows and columns, applied styles; design and layout elements.
Presentations	Text, table, image and other changes.
PDF Documents	Text, table, image and other changes.

It is quite common for document comparison software vendors to present forms of the compared document in separate windows in a GUI, where each window contains the following items, where the multiplicity of windows are displayed either on a single or on multiple computer display monitors: the original document, the modified document, and the redline (or comparison) document, with a fourth window used to present the list of changes made between document versions.

Presentation of changes made between document versions are also traditionally customizable. While the standard display of showing deletions with red underlines and additions with blue underlines is still utilized by many document comparison products, some offer the ability for users to customize the presentation of changes in the redline/comparison document.

Document comparison before the personal computer

Prior to personal computers, document comparison entailed the printing of two versions of a single document and reviewing those hard copies in detail for changes and version amendment. Included in this process were the potential for human error and the expansive administrative time necessitated by this arduous process.

First generation document comparison

With the advent of personal computers and the ubiquity of word processing software, the need to find a way to manage changes made to document versions, shared via disk or, later, email. The importance of mitigating risks associated with potential document changes became essential as the amount of document and revision sharing increased. The early, first-generation document comparison software solutions provided robust document review, checking all text in two documents for changes and presenting those changes in a third, redline or comparison version.

Second generation document comparison

As documents changed and evolved, so did document comparison solutions. The second generation of documents began utilizing tables to manage a multiplicity of document layouts. Many document comparison solutions had difficulty comparing tables in document versions. These solutions first converted tables to text arrays and then compared the created arrays. In many cases, not enough due diligence on the software’s part was conducted; users would not be informed of sections that were not successfully compared. In the second generation, Microsoft’s Track Changes option was also introduced. With Track Changes, all changes made to documents were captured and stored inside the document. Flaws in the functionality of Track Changes could render the documents unusable and some comparison offerings again had difficulty managing the complex process of comparing in a Track Changes environment.

Third generation document comparison

Prior to third generation technology, it was common for organizations to be required to utilize multiple documents for one product. A main document with various supporting documents would be utilized to present and share necessary information. However, the third generation of documents (especially Microsoft Word) enabled multiple types of information to be presented in a single document. Compound documents could include text, tables and various styles, and could also include a range of embedded objects, like Excel, Visio, ChemDraw and SmartDraw objects, and inserted images in a range of types (jpg, tiff, bmp, gif, etc.). While this enhancement greatly increased the usefulness of documents, it added an entirely new layer of risk to organizations that needed to fully understand changes made to document versions. The majority of document comparison software programs have not yet included mechanisms to mitigate the risk related to changes inside of embedded objects. The software program that can compare changes made in embedded objects provides pixel-to-pixel comparison of images and cell level redline of embedded Excel spreadsheets and other changes made to these complex, compound documents. The patent for software-based compound document comparison is held by Litéra Technology LLC.

Business Relevance

Document comparison provides a method of quality assurance. Individuals and organizations are able to assure that changes requested have been integrated properly into documents. Additionally, document comparison provides assurances that no unwarranted changes were made.

Document comparison in the legal industry

Because lawyers and legal professionals consistently share documents with opposing counsel and because the documents constructed in this business vertical may be binding, a complete mitigation of risks associated with changes made to documents is essential. If a member of opposing counsel, for example, makes one change that is not detected, such a change could exorbitantly affect a legal organization’s clients.

Document comparison in banking, finance and accounting

Because professionals in these industries manage large amounts of data in spreadsheets and because one change to a value or formula could affect an exorbitant amount of data, professionals in the banking, finance and accounting industries find document comparison (e.g., comparison of two versions of a MS Excel spreadsheet) to be extremely useful in assuring accuracy in document change management.

Creative media management and publishing

Professionals in these industries regularly work with multiple versions of single documents. Document comparison software helps these professionals ensure that all changes have been acceptably integrated into latest versions and provides them with a speedy understanding of changes made in editing and versioning of the documents they work with.