A RAND study, The Cost of Producing Electronic Documents in Civil Lawsuits, provides some insight into how much businesses are spending on electronic discovery.
It confirms that Review continues to be the most expensive stage of electronic discovery, and that most of the spending goes for work performed by outside counsel.
RAND concludes that the highest number of documents that a person can review in an hour is about 100, and it's not likely that this rate can be improved upon. Email threading; clustering; and near-duplicate detection will not necessarily allow for major improvements in review time.
The study also emphasizes the inconsistent decisions made by human reviewers. "In one case, for example, seven teams of attorneys, all trained in a similar manner and given the same instructions, examined 28,000 documents organized into 12,000 clusters to judge whether the clusters were responsive to the facts of the case. Results showed that the teams agreed only 23 to 54 percent of the time, depending on the pair of teams being compared.". It recommends predictive coding as an alternative.