/**
 * Computes the covariance between two datasets.
 * Both arrays must have the same length.
 *
 * @param {number[]} xs
 * @param {number[]} ys
 * @param {boolean} [isSample=true] - sample or population covariance. Defaults to true.
 * @returns {number}
 */
export function covariance(xs: number[], ys: number[], isSample?: boolean): number;