ptable_pop: Generate the Population Model

In power4mome: Power Analysis for Moderation and Mediation

Generate the Population Model

Description

Generate the complete population model using the model syntax and user-specified effect sizes (population parameter values).

Usage

ptable_pop(
  model = NULL,
  pop_es = NULL,
  es1 = c(n = 0, nil = 0, s = 0.1, m = 0.3, l = 0.5, si = 0.141, mi = 0.361, li = 0.51),
  es2 = c(n = 0, nil = 0, s = 0.05, m = 0.1, l = 0.15),
  es_ind = c("si", "mi", "li"),
  standardized = TRUE,
  n_std = 1e+05,
  std_force_monte_carlo = FALSE,
  add_cov_for_moderation = TRUE
)

model_matrices_pop(x, ..., drop_list_single_group = TRUE)

Arguments

model	String. The model defined by lavaan model syntax. See 'Details'.
pop_es	It can be a data frame with these columns: lhs, op, rhs, and pop. The first three columns correspond to those in a lavaan parameter table. The column pop stores the population values. The column es stores the original labels, for reference. It can also be a named character vector (named list for multigroup models) or a multiline string, which are the preferred approaches. See the help page on how to specify this vector.
es1	Set the values for each label of the effect size (population value) for correlations and regression paths. Used only if pop_es is a named vector or a multiline string. See the help page on how to specify this argument.
es2	Set the values for each label of the effect size (population value) for product term. Used only if pop_es is a named vector or a multiline string. See the help page on how to specify this argument.
es_ind	The names of labels denoting the effect size of an indirect effect. They will be used to determine the population values of the component paths along an indirect path.
standardized	Logical. If TRUE, the default, variances and error variances are scaled to ensure the population variances of the endogenous variables are close to one, and hence the effect sizes (population values) are standardized effect sizes if the variances of the continuous exogenous variables are also equal to one.
n_std	The sample size used to determine the error variances by simulation when std_force_monte_carlo is TRUE.
std_force_monte_carlo	Logical. If FALSE, the default, standardization is done analytically if the model has no product terms, and by simulation if the model has product terms. That is, error variances required to ensure implied variances equal to one are determined by simulation. If TRUE, simulation will be used whether the model has product terms or not. Always fall back to simulation if analytical standardization failed.
add_cov_for_moderation	Logical. If TRUE, the default, for a model in which one or product terms for moderation involve one or more mediator, covariances between their error terms and the product terms will be added automatically. If these covariances are not added, the model may not be invariant to linear transformation of some variables in the model.
x	It can be a 'lavaan' model syntax, to be passed to ptable_pop(), or a parameter table with the column start set to the population values, such as the output of ptable_pop().
...	If x is a model syntax, these are arguments to be passed to ptable_pop().
drop_list_single_group	If TRUE and the number of groups is equal to one, the output will be a list of matrices of one group only. Default if TRUE.

Details

The function ptable_pop() generates a lavaan parameter table that can be used to generate data based on the population values of model parameters.

Value

The function ptable_pop() returns a lavaan parameter table of the model, with the column start set to the population values.

The function model_matrices_pop() returns a lavaan LISREL-style model matrices (like the output of lavaan::lavInspect() with what set to "free"), with matrix elements set to the population values. If x is the model syntax, it will be stored in the attributes model. If the model is a multigroup model with k groups (k greater than 1), then it returns a list of k lists of lavaan LISREL-style model matrices unless drop_list_single_group is TRUE.

The role of ptable_pop()

The function ptable_pop() is used by the all-in-one function power4test(). Users usually do not call this function directly, though developers can use this function to develop other functions for power analysis, or to build their own workflows to do the power analysis.

Specify the Population Model by 'model'

Single-Group Model

For a single-group model, model should be a lavaan model syntax string of the form of the model. The population values of the model parameters are to be determined by pop_es.

If the model has latent factors, the syntax in model should specify only the structural model for the latent factors. There is no need to specify the measurement part. Other functions will generate the measurement part on top of this model.

For example, this is a simple mediation model:

"m ~ x
 y ~ m + x"

Whether m, x, and y denote observed variables or latent factors are determined by other functions, such as power4test().

Multigroup Model

Because the model is the population model, equality constraints are irrelevant and the model syntax specifies only the form of the model. Therefore, model is specified as in the case of single group models.

Specify 'pop_es' Using Named Vectors

The argument pop_es is for specifying the population values of model parameters. This section describes how to do this using named vectors.

Single-Group Model

If pop_es is specified by a named vector, it must follow the convention below.

• The names of the vectors are lavaan names for the selected parameters. For example, m ~ x denotes the path from x to m.

• Alternatively, the names can be either ".beta." or ".cov.". Use ".beta." to set the default values for all regression coefficients. Use ".cov." to set the default values for all correlations of exogenous variables (e.g., predictors).

• The names can also be of this form: ".ind.(<path>)", whether ⁠<path>⁠ denote path in the model. For example, ".ind.(x->m->y)" denotes the path from x through m to y. Alternatively, the lavaan symbol ~ can also be used: ".ind.(y~m~x)". This form is used to set the indirect effect (standardized, by default) along this path. The value for this name will override other settings.

• If using lavaan names, can specify more than one parameter using +. For example, y ~ m + x denotes the two paths from m and x to y.

• The value of each element can be the label for the effect size: n for nil, s for small, m for medium, and l for large. The value for each label is determined by es1 and es2. See the section on specifying these two arguments.

• The value of pop_es can also be set to a value, but it must be quoted as a string, such as "y ~ x" = ".31".

This is an example:

c(".beta." = "s",
  "m1 ~ x" = "-m",
  "m2 ~ m1" = "l",
  "y ~ x:w" = "s")

In this example,

• All regression coefficients are set to small (s) by default, unless specified otherwise.

• The path from x to m1 is set to medium and negative (-m).

• The path from m1 to m2 is set to large (l).

• The coefficient of the product term x:w when predicting y is set to small (s).

Indirect Effect

When setting an indirect effect to a symbol (default: "si", "mi", "li", with "i" added to differentiate them from the labels for a direct path), the corresponding value is used to determine the population values of all component paths along the pathway. All the values are assumed to be equal. Therefore, ".ind.(x->m->y)" = ".20" is equivalent to setting m ~ x and y ~ m to the square root of .20, such that the corresponding indirect effect is equal to the designated value.

This behavior, though restricted, is for quick manipulation of the indirect effect. If different values along a pathway, set the value for each path directly.

Only nonnegative value is supported. Therefore, ".ind.(x->m->y)" = "-si" and ".ind.(x->m->y)" = "-.20" will throw an error.

Multigroup Model

The argument pop_es also supports multigroup models.

For pop_es, instead of named vectors, named list of named vectors should be used.

• The names are the parameters, or keywords such as .beta. and .cov., like specifying the population values for a single group model.

• The elements are character vectors. If it has only one element (e.g., a single string), then it is the the population value for all groups. If it has more than one element (e.g., a vector of three strings), then they are the population values of the groups. For a model of k groups, each vector must have either k elements or one element.

This is an example:

list("m ~ x" = "m",
     "y ~ m" = c("s", "m", "l"))

In this model, the population value of the path m ~ x is medium (m) for all groups, while the population values for the path y ~ m are small (s), medium (m), and large (l), respectively.

Specify 'pop_es' Using a Multiline String

When setting the argument pop_es, instead of using a named vector or named list for pop_es, the population values of model parameters can also be specified using a multiline string, as illustrated below, to be parsed by pop_es_yaml().

Single-Group Model

This is an example of the multiline string for a single-group model:

y ~ m: l
m ~ x: m
y ~ x: nil

The string must follow this format:

• Each line starts with ⁠tag:⁠.

  • tag can be the name of a parameter, in lavaan model syntax format.

    • For example, m ~ x denotes the path from x to m.

  • A tag in lavaan model syntax can specify more than one parameter using +.

    • For example, y ~ m + x denotes the two paths from m and x to y.

  • Alternatively, the tag can be either .beta. or .cov..

    • Use .beta. to set the default values for all regression coefficients.

    • Use .cov. to set the default values for all correlations of exogenous variables (e.g., predictors).

• After each tag is the value of the population value:

  -nil for nil (zero),

  • s for small,

  • m for medium, and

  • l for large.

  • si, mi, and li for small, medium, and large a standardized indirect effect, respectively.

  Note: n cannot be used in this mode.

  The value for each label is determined by es1 and es2 as described in ptable_pop().

  • The value can also be set to a numeric value, such as .30 or -.30.

This is another example:

.beta.: s
y ~ m: l

In this example, all regression coefficients are small, while the path from m to y is large.

Multigroup Model

This is an example of the string for a multigroup model:

y ~ m: l
m ~ x:
  - nil
  - s
y ~ x: nil

The format is similar to that for a single-group model. If a parameter has the same value for all groups, then the line can be specified as in the case of a single-group model: tag: value.

If a parameter has different values across groups, then it must be in this format:

• A line starts with the tag, followed by two or more lines. Each line starts with a hyphen - and the value for a group.

For example:

m ~ x:
  - nil
  - s

This denotes that the model has two groups. The values of the path from x to m for the two groups are 0 (nil) and small (s), respectively.

Another equivalent way to specify the values are using ⁠[]⁠, on the same line of a tag.

For example:

m ~ x: [nil, s]

The number of groups is inferred from the number of values for a parameter. Therefore, if a tag has more than one value, each tag must has the same number of value, or only one value.

The tag .beta. and .cov. can also be used for multigroup models.

Which Approach To Use

Note that using named vectors or named lists is more reliable. However, using a multiline string is more user-friendly. If this method failed, please use named vectors or named list instead.

Technical Details

The multiline string is parsed by yaml::read_yaml(). Therefore, the format requirement is actually that of YAML. Users knowledgeable of YAML can use other equivalent way to specify the string.

Set the Values for Effect Size Labels ('es1' and 'es2')

The vector es1 is for correlations, regression coefficients, and indirect effect, and the vector es2 is for for standardized moderation effect, the coefficients of a product term. These labels are to be used in interpreting the specification in pop_es.

The role of model_matrices_pop()

The function model_matrices_pop() generates models matrices with population values, used by ptable_pop(). Users usually do not call this function directly, though developers can use this build their own workflows to generate the data.

See Also

power4test(), and pop_es_yaml() on an alternative way to specify population values.

Examples

# Specify the model

model1 <-
"
m1 ~ x + c1
m2 ~ m1 + x2 + c1
y ~  m2 + m1 + x + w + x:w + c1
"

# Specify the population values

model1_es <- c("m1 ~ x" = "-m",
               "m2 ~ m1" = "s",
               "y ~ m2" = "l",
               "y ~ x" = "m",
               "y ~ w" = "s",
               "y ~ x:w" = "s",
               "x ~~ w" = "s")

ptable_final1 <- ptable_pop(model1,
                            pop_es = model1_es)
ptable_final1

# Use a multiline string, illustrated by a simpler model

model2 <-
"
m ~ x
y ~ m + x
"

model2_es_a <- c("m ~ x" = "s",
                 "y ~ m" = "m",
                 "y ~ x" = "nil")

model2_es_b <-
"
m ~ x: s
y ~ m: m
y ~ x: nil
"

ptable_model2_a <- ptable_pop(model2,
                              pop_es = model2_es_a)
ptable_model2_b <- ptable_pop(model2,
                              pop_es = model2_es_b)

ptable_model2_a
ptable_model2_b

identical(ptable_model2_a,
          ptable_model2_b)

# model_matrices_pop

model_matrices_pop(ptable_final1)

model_matrices_pop(model1,
                   pop_es = model1_es)

power4mome documentation built on Sept. 9, 2025, 5:35 p.m.