choicer 0.2.0

Public API cleanup (breaking)

• Low-level C++ likelihood, gradient, Hessian, score, prediction, post-estimation, and Gibbs-sampler wrappers are now internal implementation details rather than exported package functions. High-level fitting, preparation, S3 post-estimation, simulation, and recovery APIs are unchanged. The expert-facing raw BLP contractions, get_halton_normals(), set_num_threads(), and thread_info() remain public. With no downstream CRAN dependencies, v0.2.0 is the least disruptive point to narrow this surface before applications depend on unstable kernel signatures.

Robust and clustered standard errors (MNL / MXL / NL)

• vcov() on a fitted MNL, MXL, or NL model gains type = and cluster = arguments for post-hoc variance recomputation without refitting (requires keep_data = TRUE): "hessian" (inverse analytical Hessian), "bhhh" (OPG), "robust" (Huber-White / WESML sandwich), and "cluster" (cluster-robust sandwich over within-cluster sums of weighted scores — use it when the same decision maker contributes several choice situations). With no arguments, vcov() returns the as-fitted variance, unchanged.
• New cluster_col= argument on run_mnlogit() / run_mxlogit() / run_nestlogit() (and the corresponding prepare_*_data() functions): supplies per-situation cluster labels at fit time and selects the new se_method = "cluster".
• All score-based variances (BHHH, robust, cluster) are now assembled in R from one per-situation score matrix, computed by new internal C++ kernels that reuse the BHHH loop bodies. The existing se_method = "sandwich" / wesml_vcov() results are unchanged (the robust meat is the w^2 special case of the shared path).
• Scope note (MXL): clustering repairs the inference, not the estimand. The MXL simulated likelihood treats each choice situation as an independent draw from the mixing distribution (cross-sectional MSL, not the panel product form), so clustered standard errors on panel data are robust to within-person dependence but do not turn the fit into a panel mixed logit. For panel random coefficients use run_hmnlogit() (person_col=).

Corrections

• blp() for an MXL fit using draws = "generate" now carries the fitted draw count, seed, and digit-permutation setting through every contraction evaluation. Previously that path could evaluate counterfactual shares with fallback draw metadata rather than the simulation design used for the fit.
• On-the-fly MXL draws now accept all 128 prime-base dimensions implemented by the generator (the prior guard rejected the 128th dimension); 129 or more random coefficients still fail early with an explicit limit.
• HMNL prediction and welfare now use a taskwise max-shifted softmax/logsum, so extreme counterfactual utilities remain finite. Counterfactual compensating variation accepts explicit non-negative task weights and matches baseline and policy choice situations by (person_col, id_col) before subtraction; it now rejects added, dropped, or substituted tasks instead of pairing them silently by sorted position.
• The documentation previously claimed that MCMC draws from the Gibbs samplers (run_mnprobit(), run_hmnlogit(), run_hmnprobit()) are bitwise reproducible regardless of the OpenMP thread count. Independent validation disconfirmed this: draws are reproducible given the seed and a fixed thread count, and across different thread counts are invariant only up to floating-point reduction-order round-off (~1e-15), not bitwise. All man pages, vignettes, and NEWS entries now state the correct guarantee.
• Documentation now distinguishes the frequentist models’ analytical derivatives from the Bayesian C++ samplers, labels run_mxlogit() as a cross-sectional simulated likelihood, and no longer directs users to WTP-space estimation or bounded/censored mixing distributions as if choicer implemented them. The README also labels mode_choice shares and welfare as sample-design quantities unless external population shares and WESML weights are supplied.
• The MNP tutorial no longer recommends unavailable user-specified starts and now states its current post-estimation boundary. HMNP documentation now makes explicit that it uses iid utility-level normal shocks rather than the full differenced-error covariance estimated by run_mnprobit().

Hierarchical Bayes (HMNL/HMNP) convergence diagnostics and multi-chain performance

• New ess(draws) (rank-normalized bulk and tail effective sample size) and mcse(draws, kind = c("mean", "median")) (Monte Carlo standard error of the mean or median), following Vehtari, Gelman, Simpson, Carpenter & Bürkner (2021, Bayesian Analysis). rhat() gains a rank = TRUE option for the same paper’s rank-normalized, folded R-hat; the default rank = FALSE reproduces the original split R-hat exactly.
• New traceplot() generic, with a choicer_hb method, for visual inspection of the b/theta/sigma_d2 chains (and user-selected delta columns).
• summary() on a choicer_hmnl/choicer_hmnp fit now prints one consolidated multi-chain diagnostics table (R-hat, ESS bulk, ESS tail, and MCSE per parameter block, plus a single worst-case summary row spanning all J delta_j alternative effects) in place of the previous bare split-R-hat printout. HMNP now always prints an explicit “conjugate — no acceptance step” line where HMNL prints its beta/delta acceptance rates. The fit-time convergence warning now checks rank-R-hat and ESS bulk across every tracked parameter, including all J delta_j columns (previously only b/theta/sigma_d2 were checked).
• run_hmnlogit() / run_hmnprobit() fits now retain all requested chains’ hierarchical draws in a new object$chains field (a list, one element per chain: b, w_vech, delta, theta, sigma_d2, plus loglik for HMNL / sigma2 for HMNP), which the multi-chain diagnostics above consume. object$draws (chain 1) and object$beta_i (chain-1-only respondent taste summaries/draws) are unchanged.
• Chains requested via chains= are sampled sequentially, and all of their hierarchical draws are retained (object$chains) for the multi-chain diagnostics above. Parallel multi-chain execution is planned for a future release.
• The keep_beta_i = "draws" memory guard on both models is now based on a measured ~1.9x Rcpp::wrap()/R-list overhead factor (the previous formula under-estimated true memory use by roughly 2x) and accounts for all requested chains; it still fails fast before the chain runs rather than after an out-of-memory blow-up.

Hierarchical Bayesian MNL and MNP (BLP-style random-effects ASCs)

• New run_hmnlogit() (class choicer_hmnl) and run_hmnprobit() (class choicer_hmnp): hierarchical Bayes discrete choice with two decoupled random-effect levels — respondent-level structural tastes beta_i ~ N(b, W) (W is K x K, no ASC columns) and a global BLP-style alternative effect delta_j = z_j' theta + xi_j, xi_j ~ N(0, sigma_d^2) (one scalar variance regardless of J; partial pooling toward the characteristics-based mean; posterior-predictive delta for alternatives outside the estimation sample). Both models carry a first-class implicit outside option (systematic utility 0 plus its own shock) that anchors the location of delta — no base alternative or sum-to-zero constraint. Panel and cross-sectional (person_col = NULL, T_i = 1) modes share one code path.
• HMNL: adaptive RW-Metropolis-within-Gibbs on the mnprobit engine pattern (single persistent OpenMP region, per-(iteration, unit) RNG streams; draws are reproducible given the seed and a fixed thread count, and invariant across thread counts only up to floating-point reduction-order round-off, ~1e-15), with a strictly serial delta sweep (the conditionals are coupled through the softmax denominators) at O(1) incremental cost per affected task; supports log-normal coordinates via rc_dist. HMNP: fully conjugate Albert-Chib augmentation in un-differenced utility space with iid N(0, sigma^2) shocks, a non-identified sigma^2 chain (parameter expansion), and per-draw scale normalization of every identified quantity.
• Priors: b ~ N(b_bar, A^-1), W ~ IW(nu, V), theta ~ N(theta_bar, A_theta^-1), and sigma_d ~ half-Cauchy(0, s_d) via the Makalic-Schmidt conjugate scale mixture (IG fallback available).
• New preps prepare_hmnl_data() / prepare_hmnp_data() (two-level person/task indexing, implicit-outside convention shared with prepare_mnl_data(), alternative-level design Z, optional control-function residual column for price endogeneity per Petrin & Train 2010) and DGPs simulate_hmnl_data() / simulate_hmnp_data().
• Post-estimation on the shared choicer_hb class, all posterior-draw based: predict() (population/individual, entry counterfactuals via the posterior-predictive delta; HMNP probabilities by deterministic 20-node 1-D Gauss-Hermite approximation), wtp() (posterior median + quantile intervals), logsum() / consumer_surplus() (HMNL-only; probit Emax is roadmapped), elasticities() / diversion_ratios() (common-random-path perturbation engine including the outside option), recovery_table(), plus rhat() (split R-hat) and ppc_shares() diagnostics.
• Two new math articles: vignettes/articles/hierarchical_mnl_math.Rmd and hierarchical_mnp_math.Rmd; recovery demos in inst/simulations/hmnl_simulation.R / hmnp_simulation.R.

WESML sandwich inference for MNL and nested logit

• run_mnlogit() and run_nestlogit() now compute the robust (Huber–White / WESML) sandwich variance V = A^{-1} B A^{-1}, at full feature parity with run_mxlogit(). Both gain se_method = "sandwich" (bread = weighted negated Hessian, meat = weight-squared OPG) and se_method = "bhhh" (ordinary outer-product-of-gradients). run_mnlogit()’s se_method therefore now accepts "hessian" (default), "bhhh", or "sandwich"; run_nestlogit()’s accepts "hessian" (default), "numeric", "bhhh", or "sandwich".
• New C++ kernels mnl_bhhh_parallel() and nl_bhhh_parallel() accumulate the weighted outer product of per-individual scores. Their per-individual score is weight-free, so passing weights = w yields the BHHH information and weights = w^2 yields the sandwich meat. The NL kernel includes the full beta/lambda/delta score blocks (singleton-nest lambdas fixed to 1 contribute no score).
• run_mnlogit(), run_nestlogit(), prepare_mnl_data() and prepare_nl_data() gain a weights_col argument: a row-level weight column is collapsed to one weight per choice situation (validated constant within id). A choice-based-sampling provenance guard auto-adopts the recorded WESML weight column and errors rather than silently fitting unweighted under a WESML label; non-uniform weights under a non-sandwich se_method emit a warning.
• Behavior change: weighted fits now emit a steering warning recommending se_method = "sandwich" when non-uniform weights are supplied with the default ("hessian") or "bhhh" method; the "bhhh" case gets a sharper message explaining that BHHH/OPG is not a valid WESML correction (its meat is w^1, not w^2). Point estimates and standard errors are unchanged.
• wesml_vcov() now dispatches on choicer_mnl and choicer_nl (in addition to choicer_mxl), returning the post-hoc sandwich variance from a fit stored with keep_data = TRUE.
• summary() for MNL and NL fits now reports the standard-error method and any WESML weighting in the printed footer.
• Added “Choice-Based Sampling and WESML Weighting” sections to the multinomial logit and nested logit derivation vignettes.

Weighting safety hardening

• Weights are now validated to be finite and strictly positive in prepare_mnl_data(), prepare_nl_data(), and prepare_mxl_data() (covering both the weights= and weights_col= paths). Zero, negative, or non-finite weights previously could slip through and silently invalidate weighted and WESML sandwich inference (weight w enters the bread, w^2 the meat); they now error with an actionable message.
• Advanced-mode fits (input_data= passed directly to run_mnlogit(), run_nestlogit(), or run_mxlogit()) that carry WESML choice_sampling provenance but resolve to uniform weights now error instead of warning. The message explains how to proceed: bake the non-uniform WESML weights into input_data via prepare_*_data(weights=/weights_col=), or strip the provenance with attr(input_data, "choice_sampling") <- NULL for a deliberate unweighted fit. Convenience-mode behavior is unchanged.

Documentation

• Added eight vignettes: a getting-started tour (“Discrete choice from data to policy, in a dozen lines”), one per model (multinomial logit, mixed logit, nested logit, Bayesian multinomial probit, hierarchical Bayes), one on choice-based sampling and WESML weights, and one on standard errors (Hessian / BHHH / robust / cluster-robust, and when to use which).
• Added the mode_choice data set: the classic Greene & Hensher intercity travel-mode choice data (210 travellers x 4 modes), in choicer’s long layout, used by the getting-started vignette. ?mode_choice and the vignettes now document that the sample is choice-based (car under-sampled): slopes and WTP ratios are unaffected in the ASC-saturated logit, while constants, shares, and surplus levels inherit the design — see the WESML vignette for the correction.
• Added a pkgdown website configuration, including the model derivation notes as “The math behind choicer” articles.
• Expanded the identification discussions across the documentation: price endogeneity and the control-function route in the getting-started model-choice section (Petrin & Train 2010; blp() as the Berry 1994 inversion), what identifies the nested-logit dissimilarity parameters, the Keane (1992) covariance-identification caveat for the multinomial probit, and the classical MSL asymptotics (fixed-S bias, S growth conditions) in the mixed logit math note.

Mixed logit — on-the-fly digit-permuted Halton draws

• run_mxlogit() gains three new arguments: draws, seed, and scramble.
  • draws = "store" (default) keeps the existing behavior: a full K_w × S × N Halton cube is pre-materialized and stored in memory.
  • draws = "generate" activates the new mode: each individual’s S draws are computed on the fly in C++ from a compact seed, eliminating the O(N) eta_draws cube. This is the recommended choice when N is large or memory is constrained.
  • scramble = "permuted" (default when draws = "generate") applies one seeded permutation per dimension and base-digit position, shared across sequence indices. This is a deterministic position-wise digit permutation, not Owen’s nested-uniform scramble, and no standard randomized-QMC unbiasedness or replicate-error guarantee is claimed. The historical value "owen" remains as a deprecated alias for compatibility. scramble = "none" reproduces the randtoolbox sequence exactly.
  • seed sets the integer master seed for the on-the-fly generator; if NULL (default), a seed is drawn from R’s RNG so set.seed() governs reproducibility. Ignored when draws = "store".
• The draws_info field on choicer_mxl objects gains three new elements: mode ("store" or "generate"), seed, and scramble. Existing code that accesses draws_info$S, draws_info$N, or draws_info$K_w is unaffected; the new fields are NULL for objects fitted before this release.
• All post-estimation generics (predict(), elasticities(), diversion_ratios(), logsum(), consumer_surplus(), vcov()) propagate the fitted draw mode automatically — no user action required.
• Default behavior (draws = "store") is bitwise unchanged.

Bayesian models

• run_mnprobit() — Bayesian multinomial probit via Gibbs sampling with data augmentation (Albert & Chib 1993; McCulloch & Rossi 1994). Runs the non-identified chain with conjugate priors and reports identified quantities normalized per draw by sigma_11. New choicer_mnp posterior object with summary() (posterior mean, SD, credible intervals), coef(), vcov(), nobs(); math note in vignettes/articles/bayesian_multinomial_probit_math.Rmd
• C++ MCMC infrastructure built from scratch (src/rng.h, src/bayes_samplers.h): xoshiro256++/splitmix64 RNG with one stream per (iteration, observation) — draws are reproducible given the seed and a fixed OpenMP thread count (invariant across thread counts only up to floating-point reduction-order round-off, ~1e-15) — plus exact truncated-normal, multivariate-normal, Wishart (Bartlett), and inverse-Wishart samplers. The truncated normal picks the cheapest exact method per region (naive normal rejection in high-mass regions, Robert 1995 exponential rejection in the tail, inverse CDF for narrow intervals)
• The Gibbs chain runs inside a single persistent OpenMP region: the latent-utility sweep and mean refresh are work-shared across choice situations, the conjugate beta/Sigma conditionals run on the master thread between lightweight barriers with hand-rolled fixed-order linear algebra (no BLAS inside the region), and the truncated-normal conditional moments are hoisted per Sigma draw. On the _benchmarks/ MNP preset this samples ~2x faster than MNP::mnp() and ~3.5x faster than bayesm::rmnpGibbs() on one thread, and scales with threads on top
• simulate_mnp_data() — probit DGP returning a choicer_sim with truth on the identified scale; recovery_table() gains a choicer_mnp method (posterior mean/SD, normal-approximation credible intervals, sigma block); parameter-recovery walkthrough in inst/simulations/mnp_simulation.R

Post-estimation

• wtp() — willingness-to-pay for MNL, MXL, and NL with analytic delta-method standard errors. For MXL, log-normal random coefficients report the median WTP under the package’s shifted log-normal parameterization; random price coefficients are rejected
• gof() — goodness of fit: McFadden pseudo R-squared (plain and adjusted) and in-sample hit rate, with "equal_shares" (default) and "market_shares" null models; now also shown in the summary() footer
• predict(..., newdata = ) — counterfactual prediction for all three models, from either a long data.frame in the fit-time format or a modified-design list (X, alt_idx, M, …); works even with keep_data = FALSE. NL fits now store nest_idx top-level to support this
• logsum() — expected maximum utility (inclusive value) per choice situation for MNL (closed form), MXL (simulated with a dedicated per-draw kernel, avoiding the Jensen bias of averaging utilities first), and NL (nested inclusive-value formula)
• consumer_surplus() — expected consumer surplus logsum / (-alpha) (Train 2009, Ch. 3) with a delta-method standard error of the mean CS for MNL; supports newdata for policy ΔCS analysis

choicer 0.1.0

Initial CRAN release.

Supported models

• Multinomial Logit (run_mnlogit()) — estimation, prediction, elasticities, diversion ratios, BLP contraction
• Mixed Logit (run_mxlogit()) — normal and log-normal random coefficients, correlated random coefficients via Cholesky, Halton draws, elasticities, BLP contraction
• Nested Logit (run_nestlogit()) — estimation with nest-specific dissimilarity (lambda) parameters

S3 class system

• Parent class choicer_fit with subclasses choicer_mnl, choicer_mxl, choicer_nl
• Standard methods: summary(), coef(), vcov(), logLik(), AIC(), BIC(), nobs(), predict()
• Classed data objects: choicer_data_mnl, choicer_data_mxl, choicer_data_nl from prepare_*_data()

Post-estimation generics

• elasticities() — methods for MNL and MXL
• diversion_ratios() — method for MNL
• blp() — BLP contraction for MNL and MXL

API

• Dual workflow for all run_*logit() functions: convenience (pass data + column names) or advanced (pass pre-prepared input_data)
• Pluggable optimizer via optimizer = "nloptr" | "optim" | <function>
• prepare_nl_data() added for nested logit data preparation

Computation

• C++ likelihoods, gradients, and analytical Hessians via Rcpp/RcppArmadillo
• OpenMP parallelization over individuals
• Log-sum-exp trick throughout for numerical stability

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