There is a correlation between ND-GAIN scores and GDP per capita. To account for this, the authors introduce the 'GDP adjusted ND-GAIN score'. This score is defined as the distance of a country's measured ND-GAIN score and its expected value based on the regression of ND-GAIN and GDP. Positive values reflect better resilience than expected. This correlation applies to vulnerability and readiness as well. For these, positive values reflect lower vulnerability and higher readiness than expected, given a certain level of GDP per capita. The year-by-year calculation of the regression and distance from the expected value allows the index to determine that changes in the relationship over time.