where the set {r ′ i } is obtained by sampling x i values from only the cluster D (ℓ) that corresponds to the selected value of t, and not from D as a whole. This would correspond to the situation in which for a day in July, meteorological variables are randomly sampled from other days in July only. Future work shall investigate whether such an approach (e.g., based on k-means clustering of D or Gaussian mixture models for (X, T )) yields more stable results, by both theoretical analyses and numerical, real-world experiments.