Natural ecosystems are complex and highly variable at multiple size scales. Because of the difficulties of accurately summarizing complexity and variability in an index number, regulators often require a reference area for comparison with a proposed or reclaimed project area. Agreement on a suitable reference area may be a requirement prior to permitting or bond-release decisions for mining and logging operations. It is common for selection of an acceptable reference area to take a long time. There is no consensus on criteria defining reference areas because of the variability among drainage basins, streams and rivers, riparian zones, and forests and woodlots so decisions tend to be subjective rather than scientific.