R² measures how much of the variation in countries' growth rates the model explains. A perfect fit is 1.0; a score of 0 means the model is no better than predicting the average growth rate for every country; negative values mean the model is worse than that baseline. The power law achieves R²≈0.42, meaning government spending share alone explains about 42% of the variation in growth across 113 countries. In cross-country macroeconomics, R² above 0.20 is considered a strong single-variable result — most economic outcomes are shaped by dozens of factors.