efa_hull: Hull method for determining the number of factors to retain

In EFAtools: Fast and Flexible Implementations of Exploratory Factor Analysis Tools

Hull method for determining the number of factors to retain

Description

Implementation of the Hull method suggested by Lorenzo-Seva, Timmerman, and Kiers (2011), with an extension to principal axis factoring. See details for parallelization.

Usage

efa_hull(
  x,
  N = NA,
  n_fac_theor = NA,
  estimator = c("PAF", "ULS", "ML"),
  gof = c("CAF", "CFI", "RMSEA"),
  eigen_type = c("SMC", "PCA", "EFA"),
  use = c("pairwise.complete.obs", "all.obs", "complete.obs", "everything",
    "na.or.complete"),
  cor_method = c("pearson", "spearman", "kendall", "poly", "tetra"),
  n_datasets = 1000,
  percent = 95,
  decision_rule = c("means", "percentile", "crawford"),
  n_factors = 1,
  estimate_control = NULL,
  ...
)

Arguments

x	matrix or data.frame. Dataframe or matrix of raw data or matrix with correlations.
N	numeric. Number of cases in the data. This is passed to efa_parallel. Only has to be specified if x is a correlation matrix, otherwise it is determined based on the dimensions of x.
n_fac_theor	numeric. Theoretical number of factors to retain. One plus the larger of this number and the number of factors suggested by efa_parallel is used as the upper bound J of factors to extract in the Hull method.
estimator	character. The estimator to use. One of "PAF", "ULS", or "ML", for principal axis factoring, unweighted least squares, and maximum likelihood, respectively. Default is "PAF".
gof	character. The goodness of fit index to use. Either "CAF", "CFI", or "RMSEA", or any combination of them. With the "PAF" estimator, only the CAF can be used as goodness of fit index. For details on the CAF, see Lorenzo-Seva, Timmerman, and Kiers (2011).
eigen_type	character. On what the eigenvalues should be found in the parallel analysis. Can be one of "SMC", "PCA", or "EFA". If using "SMC" (default), the diagonal of the correlation matrices is replaced by the squared multiple correlations (SMCs) of the indicators. If using "PCA", the diagonal values of the correlation matrices are left to be 1. If using "EFA", eigenvalues are found on the correlation matrices with the final communalities of an EFA solution as diagonal. This is passed to efa_parallel().
use	character. Passed to stats::cor() if raw data is given as input. Default is "pairwise.complete.obs".
cor_method	character. One of "pearson", "spearman", or "kendall", passed to stats::cor(). "poly" and "tetra" are not supported because HULL derives its factor-search bound from an internal parallel analysis against continuous reference data. Default is "pearson".
n_datasets	numeric. The number of datasets to simulate. Must be at least 1. Default is 1000. This is passed to efa_parallel().
percent	numeric. The percentile to take from the simulated eigenvalues. Default is 95. This is passed to efa_parallel().
decision_rule	character. Which rule to use to determine the number of factors to retain. Default is "means", which will use the average simulated eigenvalues. "percentile", uses the percentiles specified in percent. "crawford" uses the 95th percentile for the first factor and the mean afterwards (based on Crawford et al, 2010). This is passed to efa_parallel().
n_factors	numeric. Number of factors to extract if "EFA" is included in eigen_type. Default is 1. This is passed to efa_parallel().
estimate_control	an estimate_control() object with the estimation settings for the efa_fit() fits of the 0 to J factor solutions, and for the fit inside the internal efa_parallel() call. NULL (default) uses the efa_fit() defaults. This object carries estimation settings only; the fits are always unrotated, which the hull statistics (CFI, RMSEA, CAF) do not depend on.
...	Further arguments passed to efa_fit(), also in efa_parallel(). The estimation tuning knobs are not passed here; they live in estimate_control, a rotation setting is not accepted because the fits are unrotated, and neither are the standard-error arguments (se, b_boot, ci, seed), because the fits are internal steps whose standard errors are not reported.

Details

The Hull method aims to find a model with an optimal balance between model fit and number of parameters, retaining only major factors (Lorenzo-Seva, Timmerman, & Kiers, 2011). It fits 0 to J factors – where J is the number of factors suggested by parallel analysis (or n_fac_theor, if that is larger), plus one – keeps the solutions on the upper boundary of the convex hull of goodness-of-fit against degrees of freedom, and selects the one at the sharpest elbow, i.e. with the highest st value.

Because it trades fit against parsimony instead of testing against a null model of uncorrelated variables, the Hull method does not lose accuracy for the correlated-factor structures where parallel analysis (efa_parallel()) tends to under-extract; the CAF variant in particular was among the more accurate criteria in Auerswald and Moshagen (2019). It needs at least six indicators and fits a model at every candidate factor count, so it is comparatively slow and is not an option for very short scales.

The efa_parallel function and the principal axis factoring of the different number of factors can be parallelized using the future framework, by calling the future::plan() function. The examples provide example code on how to enable parallel processing.

The upper bound J comes from efa_parallel(), which compares against simulated data, so the suggested number of factors varies slightly from run to run; a criterion-based rotation passed through ... adds its own random starts. Call base::set.seed() beforehand to make a run reproducible; the result is then also independent of the parallel plan.

Note that if gof = "RMSEA" is used, 1 - RMSEA is actually used to compare the different solutions. This is necessary due to how the heuristic to locate the elbow of the hull works.

The solutions are fitted without inequality constraints, so a solution can be inadmissible (a Heywood case, or a fit that did not converge). Only the selected solution is checked for this; if it is inadmissible a warning is raised and the retained number of factors should be interpreted with caution.

The ML estimation method uses the psych::fa() starting values. See also the efa_fit documentation.

Value

An object of class efa_retention (see print.efa_retention() and plot.efa_retention() for the print and plot methods). Its main fields are:

n_factors	A named numeric vector with the suggested number of factors for each requested goodness-of-fit index ("CAF", "CFI", and/or "RMSEA").
results	A list with one record per goodness-of-fit index, each holding the goodness-of-fit values, the degrees of freedom, the hull membership, and the retained solution used for printing and plotting. Each record also carries st, the elbow sharpness of every solution (see details): the retained solution has the largest value, and the runner-up shows how close the selection was. st is NA for the solutions where it is undefined, that is for those not on the hull and for the two hull endpoints, which have no neighbouring hull solution on one side. When fewer than three solutions remain on the hull, st is undefined throughout and the whole vector is NA; the retained solution is then the one with the highest goodness of fit, and a warning says so.
settings	A list of the settings used, including n_fac_max, the upper bound J of the number of factors to extract (see details). For backwards compatibility the estimator is also repeated in settings$method.

For backwards compatibility the per-index suggestions are additionally available as the top-level fields n_fac_CAF, n_fac_CFI and n_fac_RMSEA, each NA if that index was not requested in gof. New code should read them from n_factors instead.

Source

Auerswald, M., & Moshagen, M. (2019). How to determine the number of factors to retain in exploratory factor analysis: A comparison of extraction methods under realistic conditions. Psychological Methods, 24(4), 468–491. https://doi.org/10.1037/met0000200

Lorenzo-Seva, U., Timmerman, M. E., & Kiers, H. A. (2011). The Hull method for selecting the number of common factors. Multivariate Behavioral Research, 46(2), 340-364.

See Also

efa_retain() as a wrapper function for this and the other factor retention criteria.

Other factor retention criteria: efa_cd(), efa_ekc(), efa_kgc(), efa_map(), efa_nest(), efa_parallel(), efa_retain(), efa_scree(), efa_smt()

Examples

# using PAF (this will print a message if gof is not specified manually
# and CAF will be used automatically)
efa_hull(test_models$baseline$cormat, N = 500, gof = "CAF", n_datasets = 100)

# using ML with all available fit indices (CAF, CFI, and RMSEA)
efa_hull(test_models$baseline$cormat, N = 500, estimator = "ML", n_datasets = 100)

# using ULS with only RMSEA
efa_hull(test_models$baseline$cormat, N = 500, estimator = "ULS", gof = "RMSEA",
         n_datasets = 100)


## Not run: 
# using parallel processing (Note: plans can be adapted, see the future
# package for details). future::plan() returns the plan it replaces, so
# on.exit() puts the session back as it was -- also if the call fails.
local({
  old_plan <- future::plan(future::multisession, workers = 2)
  on.exit(future::plan(old_plan), add = TRUE)
  efa_hull(test_models$baseline$cormat, N = 500, gof = "CAF")
})

## End(Not run)

EFAtools documentation built on Aug. 21, 2026, 5:16 p.m.