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How deep is your index? (And why it matters in eDiscovery) 

Indexing in eDiscovery is the process of producing a searchable catalogue of all the files, email messages and other content in a set of electronic data. Its purpose is to simplify, accelerate and enhance your ability to search that data and find exactly what you’re looking for – a considerable advantage in eDiscovery. 

Indexing can be applied to a specific dataset or an entire Microsoft 365 environment and can comprise both the metadata associated with the files and the contents of the files themselves.

The importance of indexing in eDiscovery 

Search is integral to eDiscovery. The faster, more efficient, and more accurate your search, the faster, more efficient, and more defensible your eDiscovery can be.  

Indexing not only improves search speeds, it also improves accuracy, reduces blind spots and decreases the risk of under- and over-collection. This minimises the volume of irrelevant information moving forward in the eDiscovery process, reducing processing, analysis and review timelines and associated costs. 

A comparison of indexing in eDiscovery tools

It doesn’t take much digging to reveal the sizeable upgrade in capability and complexity (at all stages of the EDRM) that Microsoft Purview eDiscovery’s Premium licence tier offers over eDiscovery (Standard). 

In terms of indexing, Purview eDiscovery (Standard) has very basic capabilities. The amount of body content (document/email/item itself) that can be indexed is limited, as are the supported file types, with no support for items like image files. 

Purview eDiscovery (Premium) has a higher capacity for body content indexing and includes support for a much wider range of file types. This includes items only partially indexed in eDiscovery (Standard) and image-only PDFs which can be converted into machine-readable (and therefore indexable) content using Optical Character Recognition (OCR).  

Needless to say, this enables a much more comprehensive – and therefore accurate – search using eDiscovery (Premium) than is possible with eDiscovery (Standard)’s entry-level functionality. 

Understanding indexing in eDiscovery is an important part of collecting and processing data . Having 'dark data' that has not been reviewed for relevance in the context of a case presents a potential blind spot that could make a big difference during ECA or later during litigation.

Indexing-related challenges in Microsoft 365 that affect eDiscovery 

No matter how powerful your indexing capabilities, every environment will inevitably have some documents that cannot be fully indexed.  

Common reasons include: 

  • Email messages with file attachments that cannot be opened. 
  • Email messages with too many file attachments. 
  • Email messages with file attachments that are too large. 
  • Supported files where indexing errors have occurred.


Unfortunately, just because an item is only partially indexed doesn’t mean it can be safely excluded from further investigation. Very often, these files will need to be re-examined to validate their relevance and return them to the searchable dataset if necessary – a process that will likely involve additional work and a specialist eDiscovery tool like Reveal. 

As such, it’s very important to select an export option that includes partially indexed items. The export should include an error tag and error properties for these items, making it easier to identify them for further review. 

Making the most of Microsoft Purview eDiscovery 

If there is one thing all specialist eDiscovery tools have in common, it’s that they are complex, nuanced, and multi-faceted. Microsoft eDiscovery (Premium) is no different. 

Attending a training course is a great way to start your eDiscovery (Premium) journey off on the right foot. However, it’s only with regular use (and, ideally, the support of a team of experienced professionals) that you can truly unlock the full value of eDiscovery (Premium)’s advanced features. This isn’t always a practical or cost-effective option, in which case Salient’s outsourced Microsoft eDiscovery service may be the answer you need. 

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