Best number of genes (N) used in age estimation and the difference of median of age is the absolute difference between the median of estimated age and the median of chronological age.

The significance of the error was determined by obtaining 1,000 randomized cross-validation errors with age information randomly shuffled; the significance of the prediction error is the fraction of the 1,000 randomized errors lower than the actual cross-validation error. The best N varies across regions.