Bivariate Within- and Between-Cluster Correlations // Separates supplied variables into within- and between-cluster components and calculates bivariate correlations for each level separately. For Pearson correlations, the centered-score decomposition corresponds to commonly used between- and within- cluster correlations reviewed by Tu et al. (2025) doi:10.1002/sim.10326. The package's descriptive Spearman option is distinct from the clustered rank parameters introduced in that paper. The package is also motivated by the distinction between within- and between-person variation described by Curran and Bauer (2011) doi:10.1146/annurev.psych.093008.100356 and by Hamaker (2024) doi:10.1080/00273171.2022.2155930. The package is intended for longitudinal or otherwise clustered data where researchers need transparent correlation matrices before fitting more complex multilevel models.