System flexibility increases as the color of the numbered boxes progresses from red to green, and as the number increases from 1 to 10. The items at the top of the table are those attributes that help efficiently integrate wind power into power systems operation. Although the table uses a simplistic 1–10 scoring system, it has proven useful as a high-level, qualitative tool. The red, yellow, and green result cells show the ease (green) or difficulty (red) that a hypothetical system would likely have integrating large amounts of wind power. RTO is regional transmission organization; ISO is independent system operator. Source: Milligan, M.; et al. Oct 22, 2013. Wind Integration Cost and Cost-Causation. 12th Annual International Workshop on Large-Scale Integration of Wind Power into Power Systems as Well as on Transmission Networks for Offshore Wind Power Plants. NREL/CP-5D00-60411. National Renewable Energy Laboratory