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stratifyR 2.0-3 (2026-10-16)

A robustness release for very heavy-tailed populations, found with a 20-seed repeat of the simulation study.

Solvers

• method = "cobyla" on data: one of the starting points now comes from a quick, coarse exact search (dynamic programming over about 400 quantile cut points, a few milliseconds). On infinite-variance samples such as a Pareto with shape 2, whose range can be hundreds of times the interquartile range, COBYLA previously missed the narrow lower strata at larger h (up to 170% above the optimum); it is now within 1% of the exact optimum in all our tests.

• method = "cobyla": random restarts now perturb the starting probabilities rather than the boundary values, and the minimum separation between boundaries is a tiny fraction of the range instead of range/1000.

• method = "global" on data now searches on the rank scale (boundaries are parameterised by their position in the sorted data), so DIRECT-L covers the lower tail as finely as the rest of the data.

• A failed COBYLA solve on extreme data could stop strata.data() with an obscure findInterval() error; it is now handled.

• When the DP recursion cannot find valid boundaries for the fitted distribution (possible on very heavy-tailed data), strata.data() now stops with a clear message suggesting method = "exact" or method = "cobyla".

• method = "dp": after the DP search on the fitted model, a short local refinement (BOBYQA) of the boundaries is tried. It is kept only when it lowers the model cost and gives valid strata, so it can never make the DP solution worse. This removes most of the error caused by the grid.

• method = "dp": two fixes in the original DP traceback. Grid positions were converted to table indices by truncation, so a value such as 3.9999999 grid steps was read as step 3; they are now rounded. And the second, finer pass only fills a window of the table around the first answer; when its answer moved to the edge of that window, earlier stages read empty cells and the solver stopped. It now keeps the first-pass answer in that case (the final refinement improves it anyway).

• method = "exact": when very large data are thinned to a candidate set, the smallest and largest values are now always kept.

Distribution fitting

• Pareto and triangular fits never succeeded inside the package (the fitting function looked for dpareto() and dtriang() on the search path), so these families were never chosen and DP used a wrong model on such data. Both are now fitted by maximum likelihood with their own code: the Pareto (Lomax) shape and scale by Nelder-Mead and BFGS on the log scale, and the triangle by profiling the likelihood over the mode with the bounds set just outside the data range.
• The Pareto log-likelihood is computed in a numerically stable form. The textbook form subtracts two huge, nearly equal numbers when the shape and scale are large; on Apple silicon the optimiser could drift there, the log-likelihood collapsed to about zero, and the Pareto then wrongly won the AIC comparison on gamma, Weibull, uniform and triangular data (and the DP then stopped).
• The Cauchy fit now uses robust starting values (median and half the IQR) and keeps the scale positive, so it no longer fails on extreme samples.
• strata.data() reports right-triangular data as "rtriangle" correctly, and fitted parameters are moved back to the data scale correctly for the Pareto and triangular families.

Stratum variances

• Pareto strata: the truncated moments are computed in a numerically stable form, which matters for shapes near 2 and wide strata.
• Triangular and right-triangular strata: exact closed-form truncated moments (split at the mode) replace the previous approximation.
• Both were checked against numerical integration to about 1e-14.

Other fixes

• method = "cobyla" and method = "global" on data: the objective rebuilt the outer limits of the range as initval * maxval and finval * maxval, which can round a hair inside the data, so the smallest or largest unit was silently left out while optimising. On data with extreme values in both tails (for example a Cauchy sample) the solvers then optimised the wrong cost and could end up far from the optimum (up to 41% in a 20-seed study). Units are now assigned with the interior boundaries only, exactly as in the final allocation, and both solvers reach the exact optimum on these samples. The same fix is applied in the interactive boundary plot.

• method = "cobyla" on data now keeps the coarse exact start as a candidate answer, so its result can never be worse than that start, and the coarse search keeps more cut points in both tails.

• method = "cobyla" and method = "global" on data now place each boundary halfway between the two data values on either side of it. Before, on data with many tied values, rounding could move the tied units at a boundary into the next stratum, so the reported WhShTot could be slightly higher than the cost the solver had found (0.2% on the math data at h = 3).

• strata.distr() now stops with a clear message when the range [initval, initval + dist] lies outside the support of a uniform or triangular distribution, instead of failing inside the allocation step.

• strata.distr(): the stratum population sizes Nh now add up to N exactly (largest remainder). Before, each N * Wh was rounded on its own, so the total could be off by one.

• summary() output inside R Markdown and Quarto documents now renders as formatted HTML. Before, the heading lines appeared as raw HTML text.

Documentation

• The vignette describes the new solver details, its comparison table was recomputed with this version, and the right-triangular and uniform examples were corrected to match what the package now fits.

Tests

• New unit tests: COBYLA within 2% of the exact optimum on an infinite-variance Pareto sample at h = 4; COBYLA and GLOBAL equal to the exact optimum on tied data; the new range check in strata.distr(); COBYLA equal to the exact optimum on a two-sided heavy-tailed sample.

stratifyR 2.0-2 (2026-10-14)

New solver

• strata.data(..., method = "exact") runs Dynamic Programming on the data themselves. It returns the partition with the smallest empirical Neyman cost over all partitions into h contiguous strata with at least two units each, never splitting tied values. It is exact for data with up to 20,000 distinct cut points (about 15 to 20 seconds for 14,000 units at h = 6); beyond that it searches 20,000 cut points spread evenly in rank, which can be changed with options(stratifyR.exact_max_cand = ). The result records this in exact_full. The "dp" solver remains the default: it is globally optimal on a fine grid for the fitted distribution.

Bug fixes (all solvers)

• The DP solver rounded its boundaries to two decimals, and the top boundary could round below the largest value, which then fell outside every stratum (for example 4,999 of 5,000 units counted). Boundaries are now kept at full precision and the top boundary is always the data maximum. Reported Neyman costs from 2.0-1 could be up to about 1 percent too low because of this.
• method = "exact" places each boundary exactly on a data value. The allocation step rebuilt it as (b / maxval) * maxval, which on some platforms (for example Apple silicon) can round one bit below b and move the boundary unit into the next stratum, so the reported cost could be a little above the true optimum (0.25% for a Cauchy sample at h = 3). The allocation step now uses a cut point halfway to the next data value.
• data.alloc() treated each stratum as closed at both ends, so a value equal to a boundary was counted in two strata. Every unit is now assigned to exactly one stratum, (b[h-1], b[h]].
• WhShTot is now reported to six decimals instead of three.
• Stratum sample sizes are now rounded with the largest-remainder rule, so they always add up to n and never exceed N_h. Rounding each n_h on its own could give a total of n + 1 or n - 1 (for example 63 + 103 + 135 = 301 for the anaemia Iron data at h = 3, n = 300).
• Every solver now returns h boundaries in OSB, the last being the top of the range.

Bug fixes (continuous solvers)

• method = "cobyla" and method = "global" no longer force every stratum to be at least (range)/(3h) wide. That rule excluded the narrow strata that skewed and heavy-tailed populations need, so on Pareto, log-normal and Cauchy data the two solvers returned designs with up to several times the optimal Neyman cost. Boundaries now only need to be range/1000 apart, as in the problem formulation.
• The degeneracy check no longer treats a narrow stratum as a failure and no longer silently switches to the DP solver. A solution is rejected only if a stratum holds fewer than two units.
• method = "global" no longer accepts a result whose objective is the infeasibility penalty (a design with empty strata). If the global phase finds no feasible point, it falls back to multi-start COBYLA.
• The random restarts no longer change the user’s random number stream.
• method = "global" no longer reports converged = FALSE when the BOBYQA refinement confirms the DIRECT-L point without improving it.

Shiny app

• The strata-count (H) selector and the “Generate R code” button no longer pass an N argument to strata.data(), which has none. Previously this made the H curve drop points and the generated code fail when a population size was set.
• Solver labels now describe GLOBAL as DIRECT-L followed by local refinement (the refinement step uses BOBYQA).
• New stratifyR hex logo in the app header (and as the browser tab icon), and the version badge now reads the installed package version instead of a fixed “2.0-1”.
• New EXACT solver option for data sources. With an assumed distribution the app uses DP, which is exact for that distribution.
• Help texts no longer describe COBYLA as unreliable on heavy tails; that was caused by the minimum-width rule fixed above.

Vignette

• Section 7 is rewritten for the four solvers (DP, COBYLA, GLOBAL and EXACT), with a guide to choosing one and worked examples that include method = "exact".
• New Section 8, “Working with the Results”: reading the output, choosing the number of strata, compare_designs(), cost-aware allocation, plots and large frames, with a worked example on household expenditure.
• The method comparison table (now Section 10) is recomputed with 2.0-2 and shows each solver’s cost against the exact optimum.
• Removed a development-only block that sourced the R files from the source tree, and updated the package details at the top.

Other

• Distribution fitting no longer prints optimiser errors and warnings to the console for families that fail to fit; those families are skipped quietly, as before.
• Added ORCID for the maintainer, and URL and BugReports fields.
• DESCRIPTION now describes the four solvers correctly (GLOBAL refines with BOBYQA, not COBYLA).
• fit_distribution() is documented as internal; it was marked for export but never exported.
• Comments in cobyla.optim.R updated to match the 2.0-2 behaviour.
• Help page for strata.data() corrected: "dp" is the default, and the "exact" method is documented.

stratifyR 2.0-1 (2026-06-14)

New features

• strata.data() and strata.distr() gain a method argument supporting three optimisation strategies:
  • "dp" (default) — the original exhaustive Dynamic Programming grid search, guaranteed globally optimal on the discrete grid.
  • "cobyla" — multi-start gradient-free local optimiser (COBYLA) via nloptr; faster than DP for large h, suitable for exploratory work.
  • "global" — two-phase global optimiser combining DIRECT-L (deterministic space-filling search) followed by COBYLA local refinement; closest to DP accuracy among the two new methods.
• New arguments n_starts, max_iter, tol, and verbose in both functions to control COBYLA/GLOBAL behaviour.
• DESCRIPTION updated: nloptr added to Imports.
• New compare_designs() function and print method. For the same total sample size it compares simple random sampling, proportional allocation and Neyman allocation, with design effects, the equivalent SRS sample size and the percentage cost saving.
• New plot() method for "strata" objects with three types: "2d" (histogram or density with boundaries and the fitted density formula), "3d" and "interactive" (both need plotly).
• New stratifyRApp() launches a self-contained shiny application: upload a CSV or Excel file or pick a built-in dataset, compute OSB and OSS with any solver, choose the number of strata, compare designs, explore the boundaries, download results and generate the equivalent R code.
• The COBYLA and GLOBAL solvers support the same cost-aware objective as DP (cost = TRUE with per-stratum costs ch).
• Solution-quality diagnostics for the COBYLA and GLOBAL solvers: a Cauchy-Schwarz optimality gap, KKT first-order residuals and a convergence flag, shown by summary().

Bug fixes

• Weibull and Gamma recurrences (critical): distr.root() and data.root() were using the unregularized upper incomplete gamma function UGamma(s, z) (via zipfR) where the regularized form Q(s, z) was required by the mathematical formulation. Fixed by replacing all calls with pgamma(z, shape = s, lower.tail = FALSE), which correctly implements Q(s, z) = Γ(s, z) / Γ(s) as defined in Abramowitz & Stegun (1972), Chapter 6, equation 6.5.2. Results for Weibull and Gamma populations are now consistent with the published theory.
• Triangular fitting in .refit_real_scale(): simultaneous MLE of all three triangular parameters (min, max, mode) produced degenerate estimates (min ≈ max ≈ mode). Fixed by fixing min and max to sample bounds and estimating only the mode via MLE, matching the approach already used in get.dist().
• get.dist(): triangular distribution now only selected over a tail model (Gamma, Weibull, Log-normal, Exponential) when its AIC advantage exceeds 10 units, preventing spurious triangular wins on mildly right-skewed data.

Vignette

• New Section 6: Alternative Optimisation Methods: COBYLA and GLOBAL — describes the motivation, algorithm, advantages, and limitations of each method, with a worked comparison example using the Anaemia/Iron data.
• New Section 8: Summary of Method Comparison Across All Supported Distributions — presents a table of DP vs COBYLA vs GLOBAL results across all ten distributions with explanation of observed patterns.
• Corrected description of Q(r, y) (regularized) vs Γ(r, y) (unregularized) in the Weibull and Gamma sections, with explicit R implementation note.
• Fixed incorrect Normal distribution example (wrong mean/sd parameters).
• Updated triangular parameter estimation example to use the fixed fix.arg approach.
• Minor fixes: removed duplicate NeedsCompilation field, removed stray \url{} in Introduction.

Backward compatibility

• All strata.data() / strata.distr() call signatures from v1.0-5 work without modification (method defaults to "dp").
• The "strata" S3 class and all slot names are unchanged.

stratifyR 1.0-5 (2026-06-04)

Initial CRAN-submission-ready version. Implemented DP solver, 10-family distribution fitting via AIC, Neyman allocation with cost support, and coloured summary.strata() output.

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