R/rma-mv.R

In clubSandwich: Cluster-Robust (Sandwich) Variance Estimators with Small-Sample Corrections

#----------------------------------------------------------------------
# utility function for computing block-diagonal covariance matrices
#----------------------------------------------------------------------

isPosDef <- function(x) {
  x_na <- is.na(x)
  mis_rows <- apply(x_na, 1, all)
  mis_cols <- apply(x_na, 2, all)
  if (all(mis_rows) | all(mis_cols)) return(TRUE)
  x_nomiss <- x[!mis_rows, !mis_cols]
  x_eig <- eigen(x_nomiss)
  all(x_eig$values > 0)    
}

check_PD <- function(vcov_list) {
  PD <- sapply(vcov_list, isPosDef)
  if (!all(PD)) {
    NPD_clusters <- names(vcov_list)[!PD]
    warn_text <- paste(c("The following clusters have non-positive definite covariance matrices:", NPD_clusters), collapse = "\n")
    warning(warn_text)
  } else {
    NULL
  }
}

#' Impute a block-diagonal covariance matrix
#'
#' @description \code{impute_covariance_matrix} calculates a
#'   block-diagonal covariance matrix, given the marginal variances, the block
#'   structure, and an assumed correlation structure. Can be used to create
#'   compound-symmetric structures, AR(1) auto-correlated structures, or
#'   combinations thereof.
#'
#' @param vi Vector of variances
#' @param cluster Vector indicating which effects belong to the same cluster.
#'   Effects with the same value of `cluster` will be treated as correlated.
#' @param r Vector or numeric value of assumed constant correlation(s) between
#'   effect size estimates from each study.
#' @param ti Vector of time-points describing temporal spacing of effects, for
#'   use with auto-regressive correlation structures.
#' @param ar1 Vector or numeric value of assumed AR(1) auto-correlation(s)
#'   between effect size estimates from each study. If specified, then \code{ti}
#'   argument must be specified.
#' @param smooth_vi Logical indicating whether to smooth the marginal variances
#'   by taking the average \code{vi} within each cluster. Defaults to
#'   \code{FALSE}.
#' @param subgroup Vector of category labels describing sub-groups of effects.
#'   If non-null, effects that share the same category label and the same
#'   cluster will be treated as correlated, but effects with different category
#'   labels will be treated as uncorrelated, even if they come from the same
#'   cluster.
#' @param return_list Optional logical indicating whether to return a list of
#'   matrices (with one entry per block) or the full variance-covariance matrix.
#' @param check_PD Optional logical indicating whether to check whether each
#'   covariance matrix is positive definite. If \code{TRUE} (the default), the
#'   function will display a warning if any covariance matrix is not positive
#'   definite.
#'
#'
#'
#' @return If \code{cluster} is appropriately sorted, then a list of matrices,
#'   with one entry per cluster, will be returned by default. If \code{cluster}
#'   is out of order, then the full variance-covariance matrix will be returned
#'   by default. The output structure can be controlled with the optional
#'   \code{return_list} argument.
#'
#' @details A block-diagonal variance-covariance matrix (possibly represented as
#'   a list of matrices) with a specified structure. The structure depends on
#'   whether the \code{r} argument, \code{ar1} argument, or both arguments are
#'   specified. Let \eqn{v_{ij}}{v-ij} denote the specified variance for
#'   effect \eqn{i}{i} in cluster \eqn{j}{j} and \eqn{C_{hij}}{C-hij} be
#'   the covariance between effects \eqn{h}{h} and \eqn{i}{i} in cluster
#'   \eqn{j}{j}. \itemize{ \item{If only \code{r} is specified,}{ each block
#'   of the variance-covariance matrix will have a constant (compound symmetric)
#'   correlation, so that \deqn{C_{hij} = r_j \sqrt{v_{hj} v_{ij},}}{C-hij =
#'   r-j * sqrt(v-hj v-ij),} where \eqn{r_j}{r-j} is the specified correlation
#'   for cluster \eqn{j}{j}. If only a single value is given in \code{r}, then
#'   it will be used for every cluster.} \item{If only \code{ar1} is
#'   specified,}{ each block of the variance-covariance matrix will have an
#'   AR(1) auto-correlation structure, so that \deqn{C_{hij} = \phi_j^{|t_{hj}
#'   - t_{ij}|} \sqrt{v_{hj} v_{ij},}}{C-hij = (ar1-j)^|t-hj - t-ij| * sqrt(v-hj
#'   v-ij),} where \eqn{\phi_j}{ar1-j} is the specified auto-correlation
#'   for cluster \eqn{j}{j} and \eqn{t_{hj}}{t-hj} and \eqn{t_{ij}}{t-ij}
#'   are specified time-points corresponding to effects \eqn{h}{h} and
#'   \eqn{i}{i} in cluster \eqn{j}{j}. If only a single value is given in
#'   \code{ar1}, then it will be used for every cluster.} \item{If both \code{r}
#'   and \code{ar1} are specified,}{ each block of the variance-covariance
#'   matrix will have combination of compound symmetric and an AR(1)
#'   auto-correlation structures, so that \deqn{C_{hij} = \left[r_j + (1 -
#'   r_j)\phi_j^{|t_{hj} - t_{ij}|}\right] \sqrt{v_{hj} v_{ij},}}{C-hij = [r-j +
#'   (1 - r-j)(ar1-j)^|t-hj - t-ij|] * sqrt(v-hj v-ij),} where
#'   \eqn{r_j}{r-j} is the specified constant correlation for cluster
#'   \eqn{j}{j}, \eqn{\phi_j}{ar1-j} is the specified auto-correlation for
#'   cluster \eqn{j}{j} and \eqn{t_{hj}}{t-hj} and \eqn{t_{ij}}{t-ij} are
#'   specified time-points corresponding to effects \eqn{h}{h} and
#'   \eqn{i}{i} in cluster \eqn{j}{j}. If only single values are given in
#'   \code{r} or \code{ar1}, they will be used for every cluster.} } If
#'   \code{smooth_vi = TRUE}, then all of the variances within cluster
#'   \eqn{j}{j} will be set equal to the average variance of cluster
#'   \eqn{j}{j}, i.e., \deqn{v'_{ij} = \frac{1}{n_j} \sum_{i=1}^{n_j}
#'   v_{ij}}{v-ij' = (v-1j + ... + v-nj,j) / n-j} for
#'   \eqn{i=1,...,n_j}{i=1,...,n-j} and \eqn{j=1,...,k}{j=1,...,k}.
#'
#' @export
#'
#' @examples
#' 
#' if (requireNamespace("metafor", quietly = TRUE)) {
#' 
#' library(metafor)
#'
#' # Constant correlation
#' data(SATcoaching)
#' V_list <- impute_covariance_matrix(vi = SATcoaching$V, cluster = SATcoaching$study, r = 0.66)
#' MVFE <- rma.mv(d ~ 0 + test, V = V_list, data = SATcoaching)
#' conf_int(MVFE, vcov = "CR2", cluster = SATcoaching$study)
#' 
#' }
#' 


impute_covariance_matrix <- function(vi, cluster, r, ti, ar1, 
                                     smooth_vi = FALSE, 
                                     subgroup = NULL, 
                                     return_list = identical(as.factor(cluster), sort(as.factor(cluster))),
                                     check_PD = TRUE) {
  
  cluster <- droplevels(as.factor(cluster))
  
  vi_list <- split(vi, cluster)
  
  if (smooth_vi) vi_list <- lapply(vi_list, function(x) rep(mean(x, na.rm = TRUE), length(x)))

  if (missing(r) & missing(ar1)) stop("You must specify a value for r or for ar1.")
  
  if (!missing(r)) {
    r_list <- rep_len(r, length(vi_list))
    if (missing(ar1)) {
      vcov_list <- Map(function(V, rho) (rho + diag(1 - rho, nrow = length(V))) * tcrossprod(sqrt(V)), 
                       V = vi_list, 
                       rho = r_list)
    }
  } 
  
  if (!missing(ar1)) {
    if (missing(ti)) stop("If you specify a value for ar1, you must provide a vector for ti.")
    
    ti_list <- split(ti, cluster)
    ar_list <- rep_len(ar1, length(vi_list))
    
    if (missing(r)) {
      vcov_list <- Map(function(V, time, phi) (phi^as.matrix(stats::dist(time))) * tcrossprod(sqrt(V)), 
                       V = vi_list, 
                       time = ti_list, 
                       phi = ar_list)
    } else {
      vcov_list <- Map(function(V, rho, time, phi) (rho + (1 - rho) * phi^as.matrix(stats::dist(time))) * tcrossprod(sqrt(V)), 
                       V = vi_list, 
                       rho = r_list, 
                       time = ti_list, 
                       phi = ar_list)
    }
    
    vcov_list <- lapply(vcov_list, function(x) {
      attr(x, "dimnames") <- NULL
      x
    })
  } 
  
  if (!is.null(subgroup)) {
    si_list <- split(subgroup, cluster)
    subgroup_list <- lapply(si_list, function(x) sapply(x, function(y) y == x))
    vcov_list <- Map(function(V, S) V * S, V = vcov_list, S = subgroup_list)
  }
  
  if (check_PD) check_PD(vcov_list)
  
  if (return_list) {
    return(vcov_list)
  } else {
    vcov_mat <- unblock(vcov_list)
    cluster_index <- order(order(cluster))
    return(vcov_mat[cluster_index, cluster_index])
  }
}


#' Impute a patterned block-diagonal covariance matrix
#'
#' @description \code{pattern_covariance_matrix} calculates a
#'   block-diagonal covariance matrix, given the marginal variances, the block
#'   structure, and an assumed correlation structure defined by a patterned
#'   correlation matrix.
#'
#' @param vi Vector of variances
#' @param cluster Vector indicating which effects belong to the same cluster.
#'   Effects with the same value of `cluster` will be treated as correlated.
#' @param pattern_level Vector of categories for each effect size, used to
#'   determine which entry of the pattern matrix will be used to impute a
#'   correlation.
#' @param r_pattern Patterned correlation matrix with row and column names
#'   corresponding to the levels of \code{pattern}.
#' @inheritParams impute_covariance_matrix
#'
#' @return If \code{cluster} is appropriately sorted, then a list of matrices,
#'   with one entry per cluster, will be returned by default. If \code{cluster}
#'   is out of order, then the full variance-covariance matrix will be returned
#'   by default. The output structure can be controlled with the optional
#'   \code{return_list} argument.
#'
#' @details A block-diagonal variance-covariance matrix (possibly represented as
#'   a list of matrices) with a specified correlation structure, defined by a
#'   patterned correlation matrix. Let \eqn{v_{ij}}{v-ij} denote the specified
#'   variance for effect \eqn{i}{i} in cluster \eqn{j}{j} and
#'   \eqn{C_{hij}}{C-hij} be the covariance between effects \eqn{h}{h} and
#'   \eqn{i}{i} in cluster \eqn{j}{j}. Let \eqn{p_{ij}}{p-ij} be the level
#'   of the pattern variable for effect \eqn{i}{i} in cluster \eqn{j}{j},
#'   taking a value in \eqn{1,...,C}{1,...,C}. A patterned correlation matrix
#'   is defined as a set of correlations between pairs of effects taking each
#'   possible combination of patterns. Formally, let \eqn{r_{cd}}{r-cd} be the
#'   correlation between effects in categories \eqn{c}{c} and \eqn{d}{d},
#'   respectively, where \eqn{r_{cd} = r_{dc}}{r-cd = r-dc}. Then the
#'   covariance between effects \eqn{h}{h} and \eqn{i}{i} in cluster
#'   \eqn{j}{j} is taken to be \deqn{C_{hij} = \sqrt{v_{hj} v_{ij}} \times
#'   r_{p_{hj} p_{ij}}.}{C-hij = sqrt(v-hj v-ij) * r[p-hj, p-ij].} 
#'   
#'   Correlations between effect sizes within the same category are defined by the diagonal
#'   values of the pattern matrix, which may take values less than one. 
#'   
#'   Combinations of pattern levels that do not occur in the patterned correlation matrix will be set equal to \code{r}.
#'   
#'   If \code{smooth_vi = TRUE}, then all of the variances within cluster
#'   \eqn{j}{j} will be set equal to the average variance of cluster
#'   \eqn{j}{j}, i.e., \deqn{v'_{ij} = \frac{1}{n_j} \sum_{i=1}^{n_j}
#'   v_{ij}}{v-ij' = (v-1j + ... + v-nj,j) / n-j} for
#'   \eqn{i=1,...,n_j}{i=1,...,n-j} and \eqn{j=1,...,k}{j=1,...,k}.
#'   
#' @export
#'
#' @examples
#' 
#' pkgs_available <- 
#'   requireNamespace("metafor", quietly = TRUE) & 
#'   requireNamespace("robumeta", quietly = TRUE)
#'   
#' if (pkgs_available) {
#' library(metafor)
#'
#' data(oswald2013, package = "robumeta")
#' dat <- escalc(data = oswald2013, measure = "ZCOR", ri = R, ni = N)
#' subset_ids <- unique(dat$Study)[1:20]
#' dat <- subset(dat, Study %in% subset_ids)
#' 
#' # make a patterned correlation matrix 
#' 
#' p_levels <- levels(dat$Crit.Cat)
#' r_pattern <- 0.7^as.matrix(dist(1:length(p_levels)))
#' diag(r_pattern) <- seq(0.75, 0.95, length.out = 6)
#' rownames(r_pattern) <- colnames(r_pattern) <- p_levels
#' 
#' # impute the covariance matrix using patterned correlations
#' V_list <- pattern_covariance_matrix(vi = dat$vi, 
#'                                     cluster = dat$Study, 
#'                                     pattern_level = dat$Crit.Cat,
#'                                     r_pattern = r_pattern,
#'                                     smooth_vi = TRUE)
#'                                     
#' # fit a model using imputed covariance matrix
#' 
#' MVFE <- rma.mv(yi ~ 0 + Crit.Cat, V = V_list, 
#'                random = ~ Crit.Cat | Study,
#'                data = dat)
#'                
#' conf_int(MVFE, vcov = "CR2")
#' 
#' }
#' 


pattern_covariance_matrix <- function(vi, cluster, pattern_level, r_pattern, r,
                                     smooth_vi = FALSE, subgroup = NULL, 
                                     return_list = identical(as.factor(cluster), sort(as.factor(cluster))),
                                     check_PD = TRUE) {
  
  if (missing(pattern_level)) stop("You must specify a vector for pattern_level.")
  if (any(is.na(pattern_level[!is.na(vi)]))) stop("The pattern_level vector cannot have missing values.")
  
  pattern_level <- as.factor(pattern_level)
  
  if (!identical(rownames(r_pattern),colnames(r_pattern))) stop("Row names of r_pattern must be identical to column names.")
  
  mat_levels <- rownames(r_pattern)
  p_levels <- levels(pattern_level)
  
  if (!all(p_levels %in% mat_levels)) {
    
    if (missing(r)) stop("At least one pattern_level is not available in r_pattern. Please specify a value for the r argument.")
    
    np_levels <- nlevels(pattern_level)
    r_pattern_full <- matrix(r, nrow = np_levels, ncol = np_levels)
    rownames(r_pattern_full) <- colnames(r_pattern_full) <- p_levels
    included_levels <- intersect(mat_levels, p_levels)
    r_pattern_full[included_levels, included_levels] <- r_pattern[included_levels, included_levels]
    r_pattern <- r_pattern_full
  } 
    
      
  cluster <- droplevels(as.factor(cluster))
      
  pattern_list <- split(pattern_level, cluster)
  
  cor_list <- lapply(pattern_list, function(x) {
    res <- r_pattern[x, x, drop=FALSE]
    diag(res) <- 1
    res
  })
  
  vi_list <- split(vi, cluster)
  
  if (smooth_vi) vi_list <- lapply(vi_list, function(x) rep(mean(x, na.rm = TRUE), length(x)))
  
  vcov_list <- Map(function(V, r_mat) r_mat * tcrossprod(sqrt(V)), 
                     V = vi_list, 
                     r_mat = cor_list)
  

  if (!is.null(subgroup)) {
    si_list <- split(subgroup, cluster)
    subgroup_list <- lapply(si_list, function(x) sapply(x, function(y) y == x))
    vcov_list <- Map(function(V, S) V * S, V = vcov_list, S = subgroup_list)
  }
  
  if (check_PD) check_PD(vcov_list)
  
  if (return_list) {
    return(vcov_list)
  } else {
    vcov_mat <- unblock(vcov_list)
    cluster_index <- order(order(cluster))
    return(vcov_mat[cluster_index, cluster_index])
  }
}

#-------------------------------------
# vcovCR with defaults
#-------------------------------------

#' Cluster-robust variance-covariance matrix for a robu object.
#' 
#' \code{vcovCR} returns a sandwich estimate of the variance-covariance matrix 
#' of a set of regression coefficient estimates from a 
#' \code{\link[metafor]{rma.mv}} object.
#' 
#' @param cluster Optional expression or vector indicating which observations 
#'   belong to the same cluster. If not specified, will be set to the factor in
#'   the random-effects structure with the fewest distinct levels. Caveat
#'   emptor: the function does not check that the random effects are nested.
#' @param target Optional matrix or vector describing the working 
#'   variance-covariance model used to calculate the \code{CR2} and \code{CR4} 
#'   adjustment matrices. If not specified, the target is taken to be the 
#'   estimated variance-covariance structure of the \code{rma.mv} object.
#' @inheritParams vcovCR
#'   
#' @return An object of class \code{c("vcovCR","clubSandwich")}, which consists 
#'   of a matrix of the estimated variance of and covariances between the 
#'   regression coefficient estimates.
#'   
#' @seealso \code{\link{vcovCR}}
#'   
#' @export
#' 
#' @examples
#' 
#' pkgs_available <- 
#'   requireNamespace("metafor", quietly = TRUE) & 
#'   requireNamespace("metadat", quietly = TRUE)
#'
#' if (pkgs_available) withAutoprint({
#' 
#' library(metafor)
#' data(dat.assink2016, package = "metadat")
#' 
#' mfor_fit <- rma.mv(yi ~ year + deltype, 
#'                  V = vi, random = ~ 1 | study / esid,
#'                  data = dat.assink2016)
#' mfor_fit
#' 
#' mfor_CR2 <- vcovCR(mfor_fit, type = "CR2")
#' mfor_CR2
#' 
#' coef_test(mfor_fit, vcov = mfor_CR2, test = c("Satterthwaite", "saddlepoint"))
#' Wald_test(mfor_fit, constraints = constrain_zero(3:4), vcov = mfor_CR2)
#' 
#' })
#' 

vcovCR.rma.mv <- function(obj, cluster, type, target, inverse_var, form = "sandwich", ...) {
  
  if (obj$withR) stop("vcovCR.rma.mv() does not work with fixed correlation matrices in the R argument.")
  
  if (missing(cluster)) {
    cluster <- findCluster.rma.mv(obj)
  } else {
    # check that random effects are nested within clustering variable
    mod_struct <- parse_structure(obj)
    
    if (length(cluster) != NROW(mod_struct$cluster_dat)) {
      cluster <- cluster[obj$not.na]
    } 
    
    nested <- test_nested(cluster, fac = mod_struct$cluster_dat)
    if (!all(nested)) stop("Random effects are not nested within clustering variable.")
  }
  
  if (missing(target)) {
    target <- NULL
    inverse_var <- is.null(obj$W)
  } else {
    if (missing(inverse_var)) inverse_var <- FALSE
  }
  vcov_CR(obj, cluster = cluster, type = type, 
          target = target, inverse_var = inverse_var, form = form)
}

# coef()
# residuals_CS()
# vcov()
# model_matrix

#-------------------------------------
# Get (model-based) working variance matrix 
#-------------------------------------

#' @export

targetVariance.rma.mv <- function(obj, cluster) {
  tV <- matrix_list(obj$M, cluster, "both")
  if (!all(sapply(tV, is.matrix))) tV <- lapply(tV, as.matrix)
  return(tV)
}

#-------------------------------------
# Get weighting matrix
#-------------------------------------

#' @export

weightMatrix.rma.mv <- function(obj, cluster) {
  if (is.null(obj$W)) {
    V_list <- targetVariance(obj, cluster)
    Wm <- lapply(V_list, function(v) chol2inv(chol(v)))
  } else{
    Wm <- matrix_list(obj$W, cluster, "both")
    if (!all(sapply(Wm, is.matrix))) Wm <- lapply(Wm, as.matrix)
  }
  return(Wm)
}

#-----------------------------------------------
# Get outer-most clustering variable
#-----------------------------------------------

get_structure <- function(obj) {
  data.frame(G = obj$withG, H = obj$withH, R = obj$withR, S = obj$withS)
}

test_nested <- function(cluster, fac) {
  
  if (is.list(fac)) {
    res <- sapply(fac, test_nested, cluster = cluster)
    return(res)
  } 
  
  groupings <- tapply(cluster, fac, function(x) length(unique(x)))
  all(groupings==1L)  
}

nest_structure <- function(x) {
  
  if (length(x) == 1) return(x) 
  
  y <- x
  for (i in 2:length(x)) {
    names(y)[i] <- paste(names(x)[1:i], collapse = "/")
    y[i] <- do.call(paste, c(x[1:i], sep = "/"))
  }
  
  y
}

parse_structure <- function(obj) {
  
  level_dat <- vector(mode = "integer")
  cluster_dat <- data.frame(row.names = 1:obj$k)
  
  if (obj$withG) {
    level_dat[["G"]] <- obj$g.nlevels[[2]]
    cluster_dat$G <- obj$mf.g$outer
  }
  
  if (obj$withH) {
    level_dat[["H"]] <- obj$h.nlevels[[2]]
    cluster_dat$H <- obj$mf.h$outer
  }
  
  if (obj$withS) {
    s_levels <- obj$s.nlevels
    names(s_levels) <- obj$s.names
    level_dat <- c(level_dat, s_levels)
    
    mf_r <- lapply(obj$mf.r, nest_structure)
    mf_all <- do.call(cbind, mf_r)
    mf_s <- mf_all[obj$s.names]
    cluster_dat <- cbind(cluster_dat, mf_s)
    cluster_dat <- droplevels(cluster_dat)
  }
  
  list(level_dat = level_dat, cluster_dat = cluster_dat)
}

#' Detect cluster structure of an rma.mv object
#' 
#' \code{findCluster.rma.mv} returns a vector of ID variables for the highest level of clustering in a fitted \code{rma.mv} model.
#' 
#' @param obj A fitted \code{rma.mv} object.
#'   
#' @return A a vector of ID variables for the highest level of clustering in \code{obj}.
#'   
#' @export
#' 
#' @examples
#' 
#' if (requireNamespace("metafor", quietly = TRUE)) {
#' 
#' library(metafor)
#' data(dat.assink2016, package = "metadat")
#' 
#' mfor_fit <- rma.mv(yi ~ year + deltype, 
#'                  V = vi, random = ~ 1 | study / esid,
#'                  data = dat.assink2016)
#'                  
#' findCluster.rma.mv(mfor_fit)
#' 
#' }
#' 

findCluster.rma.mv <- function(obj) {
  
  if (!inherits(obj, "rma.mv")) stop("`obj` must be a fitted rma.mv model.")
  
  if (obj$withR) stop("vcovCR.rma.mv() does not work with fixed correlation matrices in the R argument.")
  
  # parse model structure
  mod_struct <- parse_structure(obj) 
  
  if (length(mod_struct$level_dat) == 0L) stop("No clustering variable specified.")
  
  # determine cluster with smallest number of levels
  
  highest_cluster <- names(mod_struct$level_dat)[which.min(mod_struct$level_dat)]
  cluster <- mod_struct$cluster_dat[[highest_cluster]]
  
  # check that random effects are nested within clustering variable
  nested <- test_nested(cluster, fac = mod_struct$cluster_dat)
  if (!all(nested)) stop("Random effects are not nested within clustering")
  
  # clean up
  if (!is.factor(cluster)) cluster <- as.factor(cluster)
  droplevels(cluster)
}

#---------------------------------------
# Get bread matrix and scaling constant
#---------------------------------------

#' @export

bread.rma.mv <- function(x, ...) {

  if (inherits(x, "robust.rma")) {
    cluster <- findCluster.rma.mv(x)
    W <- weightMatrix(x, cluster = cluster)
    X_mat <- model_matrix(x)
    X_list <- matrix_list(X_mat, fac = cluster, dim = "row")
    XWX_list <- Map(function(x, w) t(x) %*% w %*% x, x = X_list, w = W)
    XWX <- Reduce(`+`, XWX_list)
  } else {
    if (is.null(x$W)) {
      B <- vcov(x) * nobs(x)
      return(B)      
    } else {
      X_mat <- model_matrix(x)
      XWX <- t(X_mat) %*% x$W %*% X_mat
    }
  }
  B <- chol2inv(chol(XWX)) * nobs(x)
  rownames(B) <- colnames(B) <- colnames(X_mat)
  return(B)
}

#' @export

v_scale.rma.mv <- function(obj) {
  nobs(obj)
}