The formula for Spearman's rank correlation coefficient is __________.

$\rho = 1 - \frac{6 \sum d_i^2}{n(n^2 - 1)}$

The formula for Spearman's rank correlation coefficient \( \rho \) is: \[ \rho = 1 - \frac{6 \sum d_i^2}{n(n^2 - 1)} \]

where:
- \( \rho \) (rho) represents Spearman's rank correlation coefficient.
- \( d_i \) represents the difference between the ranks of corresponding variables \( X \) and \( Y \).
- \( n \) is the number of pairs of ranked data points.

This formula calculates a measure of the strength and direction of association between two ranked variables, indicating how well the relationship between them can be described using a monotonic function.