The three-step approach: the measurement model is fitted first, units are assigned to classes, and only then is the outcome brought in, corrected for the fact that some assignments are wrong. Doing it in one step instead lets the outcome pull the classes toward itself; doing it naively by modal class instead attenuates every difference. This does neither.
Arguments
- x
A covariate-free fitted
multilpa()orlta()model. A fit that already uses membership covariates cannot be corrected by the fixed, unconditional classification-error matrix used here.- data
The data frame carrying the outcome, in the fit's row order. Shared fit columns are checked row by row; a frame with too little identifying information warns that alignment cannot be verified.
- outcome
Name of a numeric outcome column not used as a measurement indicator in
x. Forlevel = "groups"it must be constant within each group.- level
"individuals"relates the outcome to profiles,"groups"to group classes.- method
"bch"applies the Bolck-Croon-Hagenaars weights;"proportional"weights by the posteriors, which is simpler and attenuated;"modal"is the naive assignment, shown for comparison.- ci_level
Confidence level for the intervals.
- contrast
"none"(the default) reports one outcome mean per class."pairs"reports the difference between every pair of classes instead, which is the quantity a three-step analysis is usually run to test, with a statistic and a p-value against a difference of zero.- adjust
Multiplicity correction applied across the pairwise differences, passed to
stats::p.adjust()."BH"by default;"none"leaves the p-values uncorrected. Ignored forcontrast = "none", which tests nothing.- vcov_type
"cluster", the default, sums the influence contributions within the fit's groups and takes those groups as the independent units, which is the honest choice when the outcome is measured on observations nested inside them."independent"treats every observation as its own independent unit instead. That ignores the nesting the model was fitted to and is only defensible when there is no nesting left to ignore, such as a fit with a single group; it is never substituted silently, and the choice is recorded in the result'svcov_typeattribute.
Value
A base data.frame. For contrast = "none" it has one row per class
and the columns level, method, class, estimate, standard_error,
conf_low, conf_high and effective_n. It carries no statistic or
p_value, unlike r3step(): an outcome mean has no meaningful null value
to be tested against, and a p-value for "this class's mean is zero" would
answer a question nobody asked. The comparison that does have a null is the
difference between two classes, and that is what contrast = "pairs"
returns: one row per pair with the columns level, method, class,
reference_class, estimate (the mean of class minus the mean of
reference_class), standard_error, statistic, p_value,
p_adjusted, conf_low and conf_high, matching r3step().
A contrast with zero standard error has no defined test statistic or
p-value; those columns are NA_real_ rather than a spurious finite test.
Standard errors are cluster-robust in both shapes under the default
vcov_type = "cluster", taking the fit's groups as the independent units,
so they remain honest when the outcome is measured on observations nested
inside those groups; vcov_type = "independent" gives the unclustered
variance instead. The variance that was used is recorded in the result's
vcov_type attribute, and for contrast = "pairs" the correction applied
to p_adjusted is recorded in its adjust attribute. A
contrast = "none" table tests nothing, so it carries no adjust
attribute.
Details
The correction assumes the outcome is independent of the assigned
class given the true one, which is what makes a three-step method valid.
On a lta() fit, level = "individuals" uses one row per occasion and
relates the outcome to the profile at that occasion. The default
group-clustered variance accounts for repeated occasions within a group.
This is a marginal profile-outcome analysis; it does not estimate an
outcome effect on transitions or a sequence-level outcome model.
A cluster-robust variance is the sum of one outer product per independent
unit, and those contributions sum to zero at the estimate, so it has rank
at most one less than the number of units. With a single group it is
exactly zero and with as many groups as classes it is singular. Both are
refused with latents_too_few_groups rather than reported as a very
small standard error; vcov_type = "independent" is the labelled way to
ask for the unclustered variance instead.
The error matrix is treated as known rather than estimated, so the intervals
are optimistic and the amount is worth stating. Over 60 replications of a
two-profile design with 8% misclassification and a true class difference of
10, "bch" recovered that difference with a bias of -0.13 against -1.28 for
"modal" and -1.82 for "proportional", and its nominal 95% intervals
covered the truth 85% of the time. Treat the point estimate as close to
unbiased and the interval as somewhat too narrow; the other two methods are
biased enough that their intervals covered the truth in none of the 60.
References
Vermunt, J. K. (2010). Latent class modeling with covariates: two improved three-step approaches. Political Analysis, 18, 450–469. Bakk, Z., & Vermunt, J. K. (2016). Robustness of stepwise latent class modeling with continuous distal outcomes. Structural Equation Modeling, 23, 20–31.
Examples
set.seed(11)
group <- rep(seq_len(40), each = 10)
truth <- 1L + as.integer(runif(400) > 0.5)
example_data <- data.frame(
g = group,
a = rnorm(400, ifelse(truth == 2L, 1.1, -1.1)),
b = rnorm(400, ifelse(truth == 2L, 1.1, -1.1)),
y = rnorm(400, ifelse(truth == 2L, 10, 0))
)
fit <- multilpa(example_data, c("a", "b"), "g", n_profiles = 2,
n_group_classes = 1, n_starts = 4, seed = 1)
three_step(fit, example_data, "y")
#> Distal outcome by class (bch, 95% CI)
#>
#> Class Mean 95% CI SE Effective n
#> ----- ------ ---------------- ---- -----------
#> 1 -0.069 [-0.458, 0.321] 0.20 182.75
#> 2 9.972 [ 9.537, 10.407] 0.22 170.37
three_step(fit, example_data, "y", contrast = "pairs")
#> Distal outcome: class differences (bch, 95% CI)
#>
#> Class vs Difference 95% CI p p (BH)
#> ----- -- ---------- ------------- ----- ------
#> 2 1 10.04 [9.37, 10.71] <.001 <.001