Dynamic Factor Model

Overview

An example of how to estimate the dynamics of a latent factor. Assume that indicators are observed at two time points and you want to estimate how the period-2 value depends on the period-1 value. You need two things:

1. A measurement system that is invariant across time (same loadings, thresholds / residual SDs across the two periods) so the factor scale is comparable, and
2. A structural equation linking the period-2 factor to the period-1 factor: f₂ = α + β·f₁ + ε.

factorana supports this with a two-stage workflow, and two helper functions make the setup a few lines:

• define_dynamic_measurement() builds the Stage 1 measurement model: a 2-factor independent system (or k-factor, for k periods) with loadings and residual sigmas tied across periods via equality constraints, but measurement intercepts left period-specific.
• build_dynamic_previous_stage() converts the Stage 1 estimation result into a previous_stage object for Stage 2, carrying the anchor period’s intercepts into every factor slot. This anchors the measurement level under E[f₁] = 0 and lets the observed mean shift between periods identify the structural intercept α.

This vignette walks through a simulated example where α, β, and σ²_ε are known, shows the recovery, and explains one subtlety of the workflow (why the anchor-period intercepts are the right ones to carry into Stage 2).

Simulate data

One latent construct, three linear indicators per period, two periods. The DGP:

\[ f_1 \sim N(0,\sigma^2_{f_1}), \qquad f_2 = \alpha + \beta\,f_1 + \varepsilon, \quad \varepsilon \sim N(0,\sigma^2_\varepsilon) \]

\[ Y_{t,m} = \tau_m + \lambda_m\,f_t + u_{t,m}, \quad u_{t,m} \sim N(0,\sigma_m^2) \]

with measurement parameters τ_m, λ_m, σ_m shared across the two periods.

library(factorana)

set.seed(41)
n <- 1500

# Structural parameters (what we want to recover)
true_var_f1   <- 1.0
true_alpha    <- 0.4
true_beta     <- 0.6
true_sigma_e2 <- 0.5

# Shared measurement parameters
item_int   <- c(1.5, 1.0, 0.8)
item_load  <- c(1.0, 0.9, 1.1)   # first loading fixed to 1 in the model
item_sigma <- c(0.7, 0.75, 0.65)

f1  <- rnorm(n, 0, sqrt(true_var_f1))
eps <- rnorm(n, 0, sqrt(true_sigma_e2))
f2  <- true_alpha + true_beta * f1 + eps

gen_Y <- function(f, i) {
  item_int[i] + item_load[i] * f + rnorm(length(f), 0, item_sigma[i])
}

dat <- data.frame(
  intercept = 1,
  eval      = 1L,
  Y_t1_m1 = gen_Y(f1, 1), Y_t1_m2 = gen_Y(f1, 2), Y_t1_m3 = gen_Y(f1, 3),
  Y_t2_m1 = gen_Y(f2, 1), Y_t2_m2 = gen_Y(f2, 2), Y_t2_m3 = gen_Y(f2, 3)
)

The wide-format data has columns named <prefix><item>: Y_t1_m1, Y_t1_m2, Y_t1_m3 for wave 1, and Y_t2_m1, … for wave 2.

Stage 1: define_dynamic_measurement()

One function call builds the measurement model: a 2-factor independent system (one factor per period) with loadings and sigmas tied across periods, intercepts left free per period.

dyn <- define_dynamic_measurement(
  data                 = dat,
  items                = c("m1", "m2", "m3"),
  period_prefixes      = c("Y_t1_", "Y_t2_"),
  model_type           = "linear",
  evaluation_indicator = "eval"
)
# The wrapper generates 5 equality constraints: 2 for loadings (items
# m2, m3; item m1's loading is fixed to 1 on its factor slot) and 3
# for sigmas.
length(dyn$equality_constraints)
#> [1] 5

Estimate Stage 1 with estimate_model_rcpp() exactly as for any model system:

ctrl <- define_estimation_control(n_quad_points = 8, num_cores = 1)
result_stage1 <- estimate_model_rcpp(
  model_system = dyn$model_system,
  data         = dat,
  control      = ctrl,
  optimizer    = "nlminb",
  parallel     = FALSE,
  verbose      = FALSE
)
result_stage1$convergence
#> [1] 0

Inspect the intercepts: the wave-1 intercepts should match the DGP τ_m (because E[f₁] = 0 by convention). The wave-2 intercepts are shifted by λ_m · E[f₂], which is the mean-drift artefact we are about to discard.

est <- result_stage1$estimates
tab <- data.frame(
  m        = 1:3,
  DGP_tau  = item_int,
  wave_1   = round(c(est["Y_t1_m1_intercept"],
                     est["Y_t1_m2_intercept"],
                     est["Y_t1_m3_intercept"]), 3),
  wave_2   = round(c(est["Y_t2_m1_intercept"],
                     est["Y_t2_m2_intercept"],
                     est["Y_t2_m3_intercept"]), 3)
)
knitr::kable(tab, row.names = FALSE)

Why we do not pool the intercepts

A natural alternative is to stack period-1 and period-2 rows into one long-format dataset and fit a single-factor CFA. The resulting intercepts would be the pooled means of Y across both periods, biased by λ_m · E[f₂]/2 (because the pooled mean averages the period-1 and period-2 factor means). Plugging those into Stage 2 under-estimates α by roughly E[f₂]/2.

The wrapper avoids that: Stage 1 estimates period-specific intercepts, then build_dynamic_previous_stage() carries only the anchor period’s intercepts into Stage 2.

Stage 2: build_dynamic_previous_stage() + SE_linear

dummy <- build_dynamic_previous_stage(
  dyn           = dyn,
  stage1_result = result_stage1,
  data          = dat,
  anchor_period = 1L
)

fm_stage2 <- define_factor_model(
  n_factors        = 2,
  n_types          = 1,
  factor_structure = "SE_linear"
)
ms_stage2 <- define_model_system(
  components     = list(),       # measurement components prepended from previous_stage
  factor         = fm_stage2,
  previous_stage = dummy
)
#> Two-stage SE estimation: Stage 1 (independent) -> Stage 2 (SE_linear)
#>   Measurement parameters (loadings, thresholds) will be fixed from Stage 1
#>   Factor distribution parameters will use Stage 2 structure
#>   Fixing 16 measurement parameters, ignoring 2 factor distribution parameters

init_s2 <- initialize_parameters(ms_stage2, dat, verbose = FALSE)
init_s2$init_params["factor_var_1"]    <- unname(dummy$estimates["factor_var_1"])
init_s2$init_params["se_intercept"]    <- 0.0
init_s2$init_params["se_linear_1"]     <- 0.5
init_s2$init_params["se_residual_var"] <- 0.5

result_stage2 <- estimate_model_rcpp(
  model_system = ms_stage2,
  data         = dat,
  init_params  = init_s2$init_params,
  control      = ctrl,
  optimizer    = "nlminb",
  parallel     = FALSE,
  verbose      = FALSE
)
result_stage2$convergence
#> [1] 0

Recovery

est <- result_stage2$estimates
se  <- result_stage2$std_errors
ps  <- c("factor_var_1", "se_intercept", "se_linear_1", "se_residual_var")

tab <- data.frame(
  param = ps,
  true  = c(true_var_f1, true_alpha, true_beta, true_sigma_e2),
  est   = round(unname(est[ps]), 4),
  se    = round(unname(se[ps]),  4)
)
tab$z <- round((tab$est - tab$true) / tab$se, 2)
knitr::kable(tab, row.names = FALSE)

se_intercept (the structural α) recovers essentially exactly, which is the whole point of the workflow. The slope β, residual variance σ²_ε, and input factor variance σ²_f₁ also recover within their standard errors.

Notes on generalisation

• More than two periods. period_prefixes can have length 3, 4, or more. The wrapper builds a k-factor independent Stage 1 with all k · (n_items - 1) loadings and k · n_items sigmas tied across periods. For Stage 2 SE modelling, pick any two factors (e.g., periods t-1 and t) and build a 2-factor SE model with previous_stage.
• Ordered probit measures. Pass model_type = "oprobit" and n_categories = K. The wrapper ties the K-1 cutpoints of each item across periods (in place of sigma).
• Additional covariates. Pass covariates = c("intercept", "x1", ...). Covariate coefficients on items are estimated per-period in Stage 1 (not tied by this wrapper); the anchor-period values are carried into Stage 2.

Where to go next

• ?define_factor_model: factor_structure = "SE_quadratic" adds a quadratic term γ·f_1² to the structural equation (trap/threshold dynamics).
• ?define_model_system: the underlying equality_constraints argument, used by the wrapper.
• vignette("measurement_system", package = "factorana"): basics of measurement-system estimation.
• vignette("roy_model", package = "factorana"): a different structural setting (discrete sector choice plus continuous outcomes sharing a latent ability).