How has the study assessed compliance with WHO guidelines on green space?

The authors of the ranking have divided urban areas into 250×250 m grids. Based on previous studies they have estimated that each of the grids should have 25% of Green Area (%GA) to comply with WHO recommendation on exposure to green space.

In the case of NDVI, the authors have used mathematical models to establish which is the value that would result in compliance with the recommendations for each city.