sfa

Stochastic frontier analysis in R. A single, consistent interface to a wide range of cross-sectional, panel, latent-class, zero-inefficiency, two-tier, sample-selection, endogenous-regressor, copula and nonparametric stochastic frontier models, with a common formula syntax for modelling the variance of each error component and a common "sfareg" result object that works with the standard R modelling generics.

Beyond fitting, it provides the tools to choose among the fifteen cross-sectional inefficiency distributions rather than assume one, and to check whether the chosen specification is defensible – see Model selection and diagnostics.

Written by David H. Bernstein, Christopher F. Parmeter and Alexander D. Stead.

Installation

The released version from CRAN:

install.packages("sfa")

The development version from GitHub:

# install.packages("remotes")
remotes::install_github("davidhbernstein/sfa")

The development version here is ahead of CRAN; see NEWS.md for what has changed, including one deliberate breaking change to psfm(model_name = "TFE").

Quick start

library(sfa)

## Simulate a cross-section with known true parameters
cs <- data_gen_cs(N = 1000, rand = 1, sig_u = 0.3, sig_v = 0.3,
                  cons = 0.5, beta1 = 0.5, beta2 = 0.5, a = 4, mu = 1)

## Normal-half normal frontier
fit <- sfm(y_pcs ~ x1 + x2, model_name = "NHN", data = cs)

summary(fit)
coef(fit)              # lambda, sigma, (Intercept), x1, x2 -- see note below
logLik(fit)
head(fit$exp_u_hat)    # technical efficiency, E[exp(-u) | e]
head(fit$med_u_hat)    # median efficiency prediction (NHN only)

Which efficiency predictors come back depends on the model: exp_u_hat (Battese and Coelli 1988) is returned by most of sfm()’s models, and the Jondrow et al. (1982) point predictor u_hat = E[u | e] by NE, NTN, NU, NGE, NLN, NW, THT and tHN. See ?sfm for the full list.

A panel model, with a determinant of the inefficiency variance:

pd <- data_gen_p(t = 10, N = 100, rand = 100, sig_u = 1, sig_v = 0.3,
                 sig_r = 0.2, sig_h = 0.4, cons = 0.5, beta1 = 0.5, beta2 = 0.5)

fit_p <- psfm(y_tre_z ~ x1 + x2 | z_gtre, model_name = "TRE_Z",
              data = pd, individual = "name")

The nine entry points

All but npsfm() return an object of class "sfareg". npsfm() returns "npsfareg" instead — a kernel-estimated frontier has no parameter vector with standard errors, so coef(), vcov() and logLik() would have nothing to return.

selsfm() and ivsfm() do not take their equations through the | pipes: selsfm() takes selection and frontier as separate formulas, and ivsfm() takes formula, endogenous and instruments. Both reject a | segment, because a pipe already means “variance determinant” everywhere else and reusing it would give one character two meanings.

sfm() — cross-sectional

sfm() also offers estimator = "cols" — corrected OLS (Olson, Schmidt and Waldman 1980), closed-form and deterministic, for NHN, NE and NG — and robust divergence-based alternatives to MLE via robust = "mlqe" | "psi" | "mdpd" for NHN.

The tuning parameter of those robust criteria does not have to be guessed: hscore_select() chooses it by minimising the Hyvarinen score, calibrate_c() gives a fixed weight-matching alternative, and density_weights() shows what the estimator did to each observation. influence_sfa() reports the influence function of any fit — which observations move it, and whether the specification lets any single one of them move it without bound.

fit <- sfm(y ~ x1 + x2, data = d, model_name = "NHN")
sel <- hscore_select(fit, method = "mlqe")          # data-driven
calibrate_c(sigma_v = 0.3, sigma_u = 0.6)           # or fixed
sfm(y ~ x1 + x2, data = d, robust = "mlqe", c_mlqe = sel$c)

Any of NHN, NE and NTN can additionally take covariates in more than one error component, as named formulas rather than further pipe segments:

sfm(y ~ x1 + x2 | z_u, vhet = ~ z_v, model_name = "NHN_Z")   # heteroskedastic v
sfm(y ~ x1 + x2, muhet = ~ z_mu, model_name = "NTN")         # Battese–Coelli (1995)

vhet drives the noise scale, uhet the inefficiency scale (the same thing the | z segment does), and muhet the pre-truncation mean.

psfm() — panel

The last five are one family: each writes the firm effect as alpha_it = sum_r theta_ir * g_r(t) and reads inefficiency off it as distance from the best firm, assuming no distribution for inefficiency at all. They differ only in how much of that structure is assumed rather than estimated — SSFE fixes L = 1 with a constant basis, LS frees the basis, CSS fixes L = 3 to {1, t, t^2}, and KSS estimates both. KSS needs a balanced panel; the others do not.

GTRE_SEQ1, GTRE_SEQ2, SSFE, SSRE, SSCRE, CSS, LS and KSS are not maximum likelihood, so logLik() (and hence AIC()/BIC()) returns NA for them.

psfm_bootstrap() provides a parametric bootstrap for GTRE-family fits, parallelised over cores.

zsfm() — zero inefficiency

ZISF and ZISF_Z: a mixture of a fully efficient regime and an inefficient frontier regime, with the regime probability optionally parameterised by covariates (ZISF_Z).

lcsfm() — latent class

LCM and LCM_Z: the latent class frontier (Greene 2005; Orea and Kumbhakar 2004) — n_class unobserved technologies, each with its own frontier and its own two scales, mixed by a multinomial logit whose covariates are optional (LCM_Z). Returns posterior class probabilities and posterior-weighted efficiency alongside the class-conditional predictions.

ttsfm() — two tier

TTNE (normal–exponential–exponential), TTHN (normal–half normal–half normal), and TTNLS (nonlinear least squares, no distributional assumption beyond the means of the two one-sided components).

selsfm() — sample selection

Greene’s (2010) frontier for the case where the units in the sample are there for reasons correlated with their inefficiency, so estimating on the selected sample alone is biased. Estimated in two steps — probit, then simulated maximum likelihood — and so it takes its two equations as separate arguments rather than through pipes:

selsfm(selection = participate ~ z1 + z2,
       frontier  = y ~ x1 + x2, data = d)

ivsfm() — endogenous regressors

Amsler, Prokhorov and Schmidt (2016): one or more regressors correlated with the statistical noise. Three estimators of the same model, not three models — IVLIML (full-information maximum likelihood), IVCF (two-step control function) and C2SLS (corrected 2SLS):

ivsfm(y ~ x1 + x2, endogenous = ~ x2, instruments = ~ w1 + w2,
      data = d, model_name = "IVLIML")

With uhet the model becomes that of Amsler, Prokhorov and Schmidt (2017), in which the environmental variables entering the inefficiency scale may themselves be endogenous.

endogeneity_test() asks whether the correction was needed at all — a Wald test of H0: rho = 0, i.e. that the noise is uncorrelated with the reduced-form errors and a plain sfm() fit would already have been consistent.

fit <- ivsfm(y ~ x1 + x2, endogenous = ~ x2, instruments = ~ w1 + w2,
             data = d, model_name = "IVLIML")
endogeneity_test(fit)

copsfm() — dependence between v and u

Drops the independence assumption between the noise and inefficiency components, coupling them with a copula and integrating the resulting density by Gauss–Legendre quadrature (n_nodes).

copsfm(y ~ x1 + x2, data = d, copula = "frank")

Clayton, Gumbel and Joe carry only positive dependence. Nothing rules out a negative association between noise and inefficiency, so each also has 90 and 270 rotations that reverse the sign ("clayton270"), and 180, the survival copula, which preserves it.

Which of these actually work, measured rather than assumed. Every density here is verified three ways — it equals the second mixed partial of its own CDF, it integrates to 1 over the unit square, and it is exactly 1 at the independence parameter. That establishes the densities are right. It does not establish that the dependence parameter is recoverable, and for most of them it is not. Fitting each family to 25 samples generated from itself at n = 400:

On data generated from a Gumbel copula with Spearman rho = 0.685 at n = 2000, every family — including the true one — returns the independence boundary, and their log-likelihoods differ by less than 0.04. The likelihood is flat in the dependence parameter. This is a property of the model, not of the code.

So: prefer "frank" or "clayton", and treat the others as exploratory. copsfm() warns when you select a family that did not recover, quoting its measured collapse rate; the rotations that were never measured warn that they were not, rather than implying either outcome. More generally, the dependence parameter is estimated imprecisely even when it is recoverable — on a Gaussian design at n = 600 its sampling standard deviation is 0.385 against a truth of 0.5 — so a comparison across families is descriptive rather than evidence for a dependence structure.

npsfm() — nonparametric

Estimates the frontier by kernel regression instead of assuming it linear.

f <- npsfm(y ~ x1 + x2, data = d, method = "FLW", dist = "hn")
head(fitted(f))       # the estimated frontier
head(f$exp_u_hat)     # technical efficiency

PSZ and MY run one numerical optimization per observation — for MY, per observation per iteration — so expect them to be one to two orders of magnitude slower than FLW. npsfm() takes a single-part formula and rejects a | z segment: its heteroskedasticity is nonparametric in the covariates themselves.

Kernel regression comes from np, and SZ’s DEA step solves one linear program per unit with lpSolve. Both are in Suggests, not Imports, so they are only required if you actually call npsfm():

install.packages(c("np", "lpSolve"))

Formula syntax

Variance determinants are supplied in extra pipe-delimited segments:

y ~ x1 + x2 | z | zp

• the first segment is the frontier,
• the second parameterises the variance of the first one-sided component (sigma_u),
• the third parameterises a second one-sided component where the model has one (sigma_w in two-tier models, sigma_h in GTRE_Z).

Omitted segments default to 1, i.e. homoskedastic.

The link function differs by model family. sfm()’s NHN_Z/NE_Z and ttsfm()’s TTNE/TTHN use sigma = exp(z'delta), while psfm()’s GTRE_Z/TRE_Z use sigma = sqrt(exp(z'delta)) — that is, delta parameterises the variance rather than the standard deviation. Check which convention applies before interpreting a coefficient on z.

Model selection and diagnostics

The package offers fifteen cross-sectional inefficiency distributions. These are the tools for choosing among them, and for asking whether the choice is defensible at all.

## The score-based diagnostics differentiate the fitted likelihood, so the fit
## has to have kept it: pass keep_objective = TRUE. TIC(), vuong() and
## influence_sfa() all need this; the others do not.
fit_hn <- sfm(y ~ x1 + x2, data = d, model_name = "NHN", keep_objective = TRUE)
fit_e  <- sfm(y ~ x1 + x2, data = d, model_name = "NE",  keep_objective = TRUE)
vuong(fit_hn, fit_e)                            # neither assumed correct
influence_sfa(fit_hn)                           # who is driving this fit

spec_test_all(residuals(lm(y ~ x1 + x2, d)))    # before fitting anything
moment_range(residuals(lm(y ~ x1 + x2, d)))    # which pairs are even possible
sfma(y ~ x1 + x2, data = d, models = c("NHN", "NE", "NTN"))

## Is the assumed inefficiency distribution itself defensible? Hold the
## normality of the noise fixed and it implies a distribution for the composed
## error, so testing that is testing the assumption on u.
gof_test(fit_hn, data = d, B = 199)             # KS and Pearson chi-square

## Is there any inefficiency to speak of? The one-sided LR test is the one to
## quote: the Wald ratio and the naive LR test both have the wrong size here,
## because the null sits on a boundary.
inefficiency_test(fit_hn)

spec_test(), gof_test(), lcsfm_homogeneity() and sfma() default to a bootstrap null rather than the published asymptotic one. Their help pages give the measured size distortions behind that choice: in two cases the asymptotic limit is badly mis-sized for the models this package fits, because it is stated for a restricted specification the package does not impose.

Working with results

"sfareg" objects support the usual generics:

coef(fit); vcov(fit); logLik(fit); nobs(fit); AIC(fit); BIC(fit)
fitted(fit); residuals(fit); predict(fit, newdata = ...)
print(fit); summary(fit)

fit$out is the source of truth — a p x 3 matrix, one row per parameter, with columns par, st_err and t-val. Index it as fit$out[, "par"], never fit$out["par", ]. Its row names vary by model: several report the lambda = sigma_u/sigma_v, sigma = sqrt(sigma_u^2 + sigma_v^2) reparameterisation rather than the raw scale parameters, so read the names rather than assuming a position.

npsfm() fits are the exception. They carry no out matrix and no standard errors, so only fitted(), residuals(), nobs(), print() and summary() apply; read the frontier, its gradients and the scale estimates off the returned object ($frontier, $frontier.grad, $sigma.u, $sigma.v).

Simulating data

data_gen_cs() and data_gen_p() generate cross-sectional and panel data with known true parameters. Each returns a data frame with one response column per model family (y_pcs, y_pcs_z, y_pcs_r, y_tre_z, …), so a given model_name is matched to the column generated under its own assumptions. These generators are how the package’s estimators are checked against known truth, including npsfm()’s — NPSFM_FLW and NPSFM_SVKZ are registered in the root-n convergence framework and both pass.

Included data

Citation

citation("sfa")

Bernstein, D. H., Parmeter, C. F., and Stead, A. D. (2026). Stochastic Frontier Analysis: The sfa Package. Working Paper.

License

GPL (>= 2). See LICENSE.md.