2.2.1. Communication Systems
2.2.1.1. Email Systems 2.2.1.2. IM Systems 2.2.1.3. Audio Systems 2.2.1.4. Video Systems 2.2.1.5. Mobile Communications Systems 2.2.1.6. Collaboration Systems
Description:  Although discovery and corporate communications have been dominated by email, the emergence of Unified Messaging Systems deserves consideration when evaluating metrics of discovery. "Unified Messaging (or UM) is the integration of different streams of communication (e-mail, SMS, Fax, voice, video, etc.) into a single, or, unified 'message store', accessible from a variety of different devices. Unified messaging is a subset of a fully integrated Unified communications system." http://en.wikipedia.org/wiki/Unified_messaging.
Metric:  The metric here is the time and effort required to identify potentially relevant systems, define relevance criteria specific to the search/retrieval capabilities and extract potential volume/counts of ESI. This may require purely IT and attorney time, but many systems may require outside search technology to effect a diligent effort.