NEWS

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Synth 1.2-0

Bug fixes (vs the unreleased 1.2-0 RC1 from earlier in this development cycle)

• synth_inference() and generate_placebos() previously classified any plot-horizon year not in time.optimize.ssr as post-treatment. This silently mixed pre-treatment plot years before the SSR window (e.g. 1955-1959 in the Basque example with time.optimize.ssr = 1960:1969) into the post-period denominator, inflating post_mspe, mspe_ratio, and the mspe_test() p-value. Both now classify post as time.plot >= treatment_time, with the default treatment_time = max(time.optimize.ssr) + 1. A new treatment_time argument lets users override.

• synth_inference(method = "conformal") now uses the order statistic at rank k = ceiling((n + 1) * (1 - alpha)) instead of quantile(.., 1 - alpha, type = 1). The former is the rank that delivers exact finite-sample (1 - alpha) coverage under exchangeability; the latter selects rank ceiling(n * (1 - alpha)) and slightly under-covers. When n is too small for the requested coverage, the function now warns and returns Inf for conformal_q instead of silently using the maximum residual.

New arguments wired through synth() and generate_placebos()

• synth() now accepts cvxr_pars and torch_pars lists for tuning the quadopt = "cvxr" and quadopt = "torch" backends (e.g. torch_pars = list(device = "mps") for Apple Silicon GPU). These were supported internally by .solve_w() but were not previously exposed at the public API.

• generate_placebos() now exposes genoud, cvxr_pars, torch_pars, and treatment_time. Match these to the configuration that produced the real fit so placebos use the same optimizer and the same post-period definition.

New: post-period inference

• synth_inference() returns a prediction band around the synthetic counterfactual. Two methods are available:

  • method = "conformal" (default) — split-conformal intervals (Chernozhukov, Wuthrich, Zhu 2021), finite-sample valid under exchangeability of pre-period residuals. Half-width is the (1 - alpha)-quantile of |gap_pre|.
  • method = "parametric" — Gaussian-residual intervals. Half-width is qnorm(1 - alpha/2) * sd(gap_pre).

  Output is an S3 object of class c("synth_<method>", "synth_inference") with print() and plot() methods. The plot() method overlays the band on the treated and synthetic series.

• generate_placebos(), mspe_test(), mspe_plot(), plot_placebos() implement the in-space placebo workflow from Abadie, Diamond, and Hainmueller (2010). generate_placebos() swaps each donor into the treated slot, refits synth(), and returns a synth_placebos object. mspe_test() returns a one-sided p-value via the empirical rank of the treated unit's post/pre MSPE ratio. The function names match those in the SCtools package by design — migration is a verbatim rename and you can namespace-qualify (e.g., Synth::generate_placebos) if both packages are loaded.

• No new package dependencies. Optional parallel = TRUE in generate_placebos() uses parallel::mclapply on non-Windows.

Validity caveats

• Both prediction-interval methods produce constant-width bands and do not separately quantify in-sample uncertainty about the synthetic weights. Users who need period-varying intervals or in-sample/ out-of-sample uncertainty decomposition should see the scpi package (Cattaneo, Feng, Palomba, Titiunik).
• Conformal validity is exact under exchangeability of pre-period residuals; parametric validity assumes i.i.d. Gaussian residuals. Both are approximate when outcomes are autocorrelated.

Optional alternative QP backends

• synth() and fn.V() gain two new opt-in values for quadopt:

  • quadopt = "cvxr" solves the W-step via the CVXR package (default solver: OSQP, a hard Imports of CVXR and therefore always available; alternative solvers "SCS", "ECOS", or "MOSEK" can be selected via cvxr_pars). Adds no required dependency; CVXR lives in Suggests: and is loaded only when requested.
  • quadopt = "torch" solves the W-step via Frank-Wolfe simplex least squares using the torch package, with optional GPU/MPS support (torch_pars = list(device = "cuda") or "mps"). Also in Suggests:. The first use of torch in a session may require torch::install_torch() to download libtorch.

  quadopt = "ipop" remains the default and produces output identical to <= 1.1-10. The new backends agree with ipop on the canonical examples to within solver tolerance and exist for users with larger panels who prefer modern convex-optimization solvers (CVXR) or autodiff/GPU machinery (torch). See the ?synth Details section and the inference vignette for guidance on choosing a backend.

• New quadopt_inner and quadopt_outer arguments on synth() and generate_placebos() let users pick different backends for the two QP stages. The V-search calls fn.V() hundreds of times via optimx; running CVXR or torch on every call is much slower than ipop. Setting quadopt_outer = "cvxr" (or "torch") with quadopt left at "ipop" gives ipop's speed for the V-search and uses the modern solver only for the single final W solve. Existing scripts that pass quadopt = ... are unchanged: it now sets both quadopt_inner and quadopt_outer.

First S3 dispatch in the package

• This release introduces S3 method dispatch (print, plot) for the new synth_inference and synth_placebos classes. Existing functions (synth, dataprep, path.plot, gaps.plot, synth.tab, etc.) are unchanged in behavior and signature.

Synth 1.1-10 (2026-04-29)

Bug fixes

• synth() and fn.V(): quadopt = "LowRankQP" no longer falls through to undefined-variable code. The deprecation message is now raised via stop() rather than cat(), which matches the user's expectation that the function fails fast on an unsupported option.

• dataprep(): the missing-data warning loop in the X0 (control predictors) section now iterates over nrow(X0) instead of nrow(X1). Previously it covered only the first length(time.predictors.prior) rows, missing roughly (n_controls - 1) / n_controls of the cells.

• synth(): the error message for ncol(Z0) < 2 no longer mentions "specify only one treated unit" (it is checking for at least two control units).

• path.plot(): Ylim is now padded by a fraction of the data range, not by 0.3 * Y.min. The old formula gave 0.7 * Y.min for negative Y.min, which moved the lower bound toward zero and cropped the bottom of negative-valued series.

Quiet by default

• synth(): messages such as "X1, X0, Z1, Z0 all come directly from dataprep object", "searching for synthetic control unit", and the final summary block printing MSPE (LOSS V) and the solution vectors are now gated by verbose = TRUE. With the default verbose = FALSE, synth() runs silently. Set verbose = TRUE to restore the previous chatter.

Documentation / cleanup (no user-visible change)

• Removed dead commented-out collinearity check in synth.R.
• Removed an unreachable is.vector(X0.scaled) defensive branch in synth.R (ncol(X0) < 2 errors out earlier).
• Several typos fixed in error messages: "\ Please..." → "\n Please...", "specificy" → "specify", "on time period" → "one time period", "at least on predictor" → "at least one predictor", "variabale" → "variable", "mispecified" → "misspecified", "supoorted" → "supported", "synthtic" → "synthetic".
• synth() and fn.V(): cleaned up the unsupported-quadopt branch (also clarifies error messages).