Within- and Between-Group Correlation Methods

This vignette documents, in more detail, how within_between_correlations() (and, through it, mldesc()) estimates within-group and between-group correlations and tests them for significance, and how to decide which method and settings are appropriate for a given data set. It assumes you are already familiar with the basic idea of within-group and between-group relationships; see the “Getting Started” vignette for an introduction.

Three methods are available via the method argument: "decomposition" (the default), "sem", and "bayes".

The decomposition method

method = "decomposition" (the default) follows the variance-decomposition approach described by Pedhazur (1997, ch. 16, p. 679). For a variable \(Y\) observed for individual \(i\) in group \(j\), the total sum of squares can be split into a between-group and a within-group part:

\[\sum_{i,j}(Y_{ij} - \bar{Y})^2 = \sum_j n_j(\bar{Y}_j - \bar{Y})^2 + \sum_{i,j}(Y_{ij} - \bar{Y}_j)^2\]

where \(\bar{Y}_j\) is the mean of group \(j\) and \(\bar{Y}\) is the grand mean. The same decomposition applies to the sum of cross-products of two variables \(X\) and \(Y\). From these between- and within-group sums of squares and cross-products, two correlations can be computed:

The within-group correlation (\(r_w\)) is the correlation between the group-mean-centered deviation scores, \((X_{ij} - \bar{X}_j)\) and \((Y_{ij} - \bar{Y}_j)\). It describes how \(X\) and \(Y\) relate to each other inside groups, after removing all between-group differences.
The between-group correlation (\(r_b\)) is the correlation between the group means, \(\bar{X}_j\) and \(\bar{Y}_j\). It describes how groups that score higher on \(X\) tend to score on \(Y\).

Weighted vs. unweighted between-group correlations

Pedhazur’s between-group sum of cross-products (eq. 16.5, p. 680) weights each group’s contribution by its size, \(n_j\). within_between_correlations() reproduces this when weight = TRUE (the default): the between-group correlation is computed on the group means after replicating each one once per observation in that group, which is mathematically equivalent to a sample-size-weighted correlation of the group means. With weight = FALSE, every group counts once, regardless of size.

For balanced data (equal group sizes), the two give identical results. For unbalanced data, Snijders and Bosker (2012, sec. 3.6.2) recommend weighting the between-group correlation by \(n_j\) specifically to recover the right population-level estimate, so weight = TRUE is the better default for the point estimate when group sizes differ. (See “Choosing a method” below for why this does not change how the correlation is tested for significance.)

Significance testing

Within-group correlation. Centering each variable on its group mean is mathematically equivalent to controlling for group membership, represented as \(n_{groups} - 1\) dummy variables, in a regression of \(Y\) on \(X\). Pedhazur (1997, p. 182) notes the general principle that “testing the significance of a partial correlation coefficient is tantamount to testing the significance of the semipartial correlation, or the regression coefficient, corresponding to it” — and the significance test for a regression coefficient depends on the model’s residual degrees of freedom. Snijders and Bosker (2012, sec. 6.1) give the relevant rule for a level-one coefficient estimated alongside \(r\) other predictors in the fixed part of the model: \(df = M - r - 1\), where \(M\) is the total number of level-one observations. Substituting \(M = N\) (total observations) and \(r = n_{groups}\) (the \(n_{groups} - 1\) group dummies, plus the slope of interest itself) gives the degrees of freedom used for the within-group correlation test:

\[df_w = N - n_{groups} - 1\]

Between-group correlation. The between-group correlation is tested as an ordinary correlation among the \(n_{groups}\) group means, with \(df = n_{groups} - 2\). This holds regardless of weight: weighting the point estimate by group size changes how much influence each group has on the estimate, but it does not change the number of independently observed groups the data provide. Because of this, the significance test always uses the unweighted correlation of the group means, even when weight = TRUE and the displayed estimate is the size-weighted version. (Without this adjustment, the test of a size-weighted estimate can substantially overstate precision when group sizes are very unequal, treating data dominated by a few large groups as if it carried as much information as many similarly sized ones would.)

The SEM method

method = "sem" fits a two-level structural equation model using lavaan::sem(), with the grouping variable as the cluster variable. Rather than decomposing observed scores and correlating the resulting deviation scores and group means, this method estimates the within-group and between-group covariance matrices directly and simultaneously, using robust maximum likelihood (MLR; Hox, Moerbeek, & van de Schoot, 2018, ch. 14, sec. 14.3). The standardized solution provides the within- and between-group correlations, and significance is based on the resulting z-tests.

Because MLR estimation incorporates each group’s sample size into the likelihood automatically, there is no separate weight argument for this method: unequal group sizes are handled natively by the estimation procedure itself, rather than by an explicit weighting choice. Missing data are handled by listwise deletion (cases with any missing value on the modeled variables are excluded before estimation).

Variables that never vary within a group (e.g., a stable trait measured once per person but attached to every observation) cannot contribute to a within-group covariance and are modeled only at the between-group level; variables with an intraclass correlation near zero are modeled only at the within-group level. Correlations that cannot be estimated at a given level are reported as NA.

The Bayesian method

method = "bayes" mirrors method = "decomposition" conceptually — the within-group correlation is estimated from group-mean-centered deviation scores, and the between-group correlation from group means — but estimates both via Bayesian multivariate models fit with brms::brm() (Bürkner, 2017) instead of closed-form formulas. Rather than a point estimate and a p-value, each correlation is reported as a posterior median together with a credible interval (CI): a correlation is starred when its CI excludes zero. This requires the brms package, which is not installed with mlstats by default.

The weight argument works as it does under method = "decomposition": with weight = TRUE (default), the between-group point estimate comes from a model fit on group means replicated once per observation in that group (implicitly weighting by group size). Unlike the point estimate, the credible interval is always computed from a model fit on unique group means only, so that uncertainty reflects the actual number of groups rather than the total sample size — mirroring how the decomposition method’s significance test always uses the unweighted group means (see above). With weight = FALSE, both the point estimate and the CI come from the unweighted model. The width of the credible interval is controlled by the ci argument (default 0.9, a 90% CI).

Choosing a method

	decomposition	sem	bayes
Speed	Fast; closed-form	Slower; iterative MLR estimation	Slowest; MCMC sampling
Balanced group sizes	Works well	Works well	Works well
Very unequal group sizes	`weight = TRUE` corrects the point estimate; significance test always uses unweighted group means (see above)	Handled natively by ML for both point estimate and significance test	`weight = TRUE` corrects the point estimate; CI always uses unweighted group means (see above)
Missing data	Pairwise deletion per variable pair	Listwise deletion across all modeled variables	Pairwise deletion per variable pair
Small number of groups	Significance tests are exact	Asymptotic MLR standard errors; can be unreliable with few groups	Posteriors do not rely on asymptotic or bivariate-normal assumptions
Uncertainty	p-values	p-values (z-tests)	Credible intervals
Interpretability	Simple, transparent formulas	Estimates come from a fitted latent-variable model	Estimates come from a fitted Bayesian model

As a starting point: use method = "decomposition" (the default) for most applications, particularly when group sizes are reasonably similar or the number of groups is small. Consider method = "sem" when group sizes are very unequal and you want the significance tests — not just the point estimate — to fully account for that imbalance. Consider method = "bayes" when the number of groups is small and you want credible intervals instead of p-values, and are willing to accept longer computation times.