Chatterjee’s “rank correlation”


December 28, 2021

Sourav Chatterjee proposed a new correlation coefficient. Different from the classical correlation coefficient (e.g., Pearson’s) that can only capture linear relationship, the Chatterjee’s coefficient can indicate if Y is related to any function of X, whether it is linear or nonlinear. Suppose Y_i = X_i^2 for all i, then Y_i is determined by X_i. Pearson’s coefficient will yield very low coefficient since there was little to no linear correlation found. Meanwhile, Chatterjee’s coefficient will still have high value because it knows that Y_i depends on X_i by some function (in this case, quadratic).

This coefficient value is between 0 and 1. Meaning, the relationship is weak if the coefficient value gets closer to 0 and strong as it gets closer to 1. It is 0 iff X and Y are independent, and is 1 iff Y is a measurable function of X almost surely.

given pairs of i.i.d. random variables (X_i, Y_i) where i=1, ..., n. Chatterjee’s correlation is obtained through following procedure:

It can be implemented as a python function in few lines of code:

import numpy as np

def chatterjee(x, y):
    # sort x and get the sorted index
    idx = np.argsort(x.ravel())

    # rearrange y based on idx
    y_ = y.ravel()[idx]

    # calculate a list of ranks
    r = (y_[:, None] <= y_).sum(1)

    # the coefficient calculation
    xi = 1 - (3 * np.sum(np.abs(r[1:] - r[:-1]))) / (len(y_) ** 2 - 1)

    return xi

I compared the classical linear correlation coefficient, i.e., Pearson’s r-score (r), with Chatterjee’s xi-score (\xi). It is interesting to see in a glimpse that xi-score can capture relationship between X and Y, where the function of X is not necessarily linear. For example, the last example, i.e., quadratic function.

Linear relationship

Random scatter

Quadratic relationship

Note that although in the first example Y=X, the xi-score is less than one! This is one of the caveats of Chatterjee’s xi-score. Chatterjee himself put a remark:

“(9) If there are no ties among the Y_i’s, the maximum possible value of \xi_n(X, Y) is (n -2) / (n + 1), which is attained if Y_i = X_i for all i. This can be noticeably less than 1 for small n.”