Which of the following can affect the relevance rate observed during a review?

The relevance rate observed during a review can significantly be influenced by the volume of new documents added. When there is an influx of new documents, it can lead to a dilution in the consistency and context of previously reviewed or coded materials. As more documents are introduced into the pool, it can impact the reviewers' ability to maintain a clear understanding of what is relevant, as they must adapt to the added context and information. This can result in discrepancies in how documents are categorized based on relevance, ultimately affecting the overall relevance rate.

The other factors mentioned, while they can have some degree of influence on workflow or efficiency, do not have as direct an impact on the relevance rate itself. For example, project complexity can affect the reviewers’ workload and decision-making, but it does not inherently change the qualitative aspect of what is considered relevant among the documents. The time of day might influence the reviewers' focus or fatigue but lacks the direct correlation with relevance rate shifts. Type of coding performed pertains more to the methods used during the review rather than the actual impact on relevance assessments caused by fluctuating document volumes. Thus, the volume of new documents added stands out as the factor most directly affecting the relevance rate observed during a review.