Shape-recognition sensitivity study • janusplot

janusplot() assigns every fitted smooth to one of 24 shape categories via a (n_turning_points, n_inflections) dispatch with additional (monotonicity_index, convexity_index) disambiguation for the monotone cases (see the janusplot vignette for the full definition of the indices). How reliably does this classifier recover the ground-truth shape of a noisy sample? This vignette answers the question with a full-factorial sensitivity sweep.

Design

For each combination of ground-truth shape, sample size n, and noise level sigma, the sweep:

Generates n points from the noiseless canonical curve on x ∈ [0, 1], with y normalised to [0, 1] so that sigma is the fraction of y-range that Gaussian noise contributes – an SNR-comparable scale across shapes.
Fits mgcv::gam(y ~ s(x), method = "REML").
Classifies the fit via janusplot_shape_metrics().
Records correctness at the fine (24-category) and archetype (7-family) levels.

The design factors are orthogonal and replicated. See ?janusplot_shape_sensitivity for the function surface. The 14 canonical ground-truth shapes cover five of the seven archetypes (chaotic and degenerate have no realistic deterministic generator).

library(janusplot)
library(ggplot2)

janusplot_shape_sensitivity_shapes()
#>  [1] "linear_up"    "linear_down"  "convex_up"    "concave_up"   "convex_down" 
#>  [6] "concave_down" "s_shape"      "u_shape"      "inverted_u"   "skewed_peak" 
#> [11] "broad_peak"   "wave"         "bimodal"      "bi_wave"

Pre-registered hypotheses

Four hypotheses were pinned before running the sweep:

H1. At n = 500, sigma = 0.05, archetype accuracy exceeds 0.90 for every shape.
H2. Fine-category accuracy exceeds 0.75 at n = 500, sigma = 0.05 for monotone + unimodal shapes; wave and multimodal tolerate less noise.
H3. Rippled variants require n >= 200 and sigma <= 0.10 to resolve.
H4. At sigma = 0.40, archetype accuracy collapses below 0.50 for all but the simplest shapes.

Verdicts, evaluated against the precomputed demo below (n = 500, sigma = 0.05 for H1/H2; the demo’s grid runs sigma up to 0.40, which covers H4):

H1 – refuted. bimodal, concave_up, inverted_u and u_shape all hit 1.00, linear_up reaches only 0.70, and wave scores 0.00 at every replicate. The wave failure is not a noise-tolerance property of the classifier: the shipped wave ground-truth generator (.shape_sensitivity_generators()) has one interior turning point on x in [0, 1], one short of what the taxonomy’s own wave definition requires, so every fit is systematically dispatched to the wrong archetype regardless of noise. This is a tracked defect in the generator, not evidence about recognition difficulty – treat the wave row of every figure below as unreliable until the generator is fixed, and do not read the other five shapes’ figures as “wave included and passing”.
H2 – partly supported, confounded by the same defect. The four clean shapes clear 0.75 easily; linear_up (0.70) falls just short, which the sweep’s design does correctly flag as a boundary case rather than a clear pass; wave’s 0.00 cannot speak to the hypothesis for the reason above.
H3 – untestable with the shipped machinery. No rippled_* generator exists among the canonical shapes exercised here, so the hypothesis has no data to be adjudicated against in this vignette.
H4 – refuted, in the other direction. At sigma = 0.40 archetype accuracy stays at 0.60 or above for every shape except wave (still stuck at 0.00, for the generator reason above) – well clear of the predicted sub-0.50 collapse. Setting wave aside, the classifier is markedly more noise-robust than pre-registered.

Precomputed demo

The package ships a small-footprint precomputed sweep – 6 shapes across 5 of the 7 archetypes (u_shape and inverted_u both roll up to unimodal) × 3 sample sizes × 4 noise levels × 30 replicates = 2160 fits – so you can explore the API without running the full sweep yourself.

data("shape_sensitivity_demo")
str(shape_sensitivity_demo, vec.len = 2)
#> 'data.frame':    2160 obs. of  14 variables:
#>  $ truth             : chr  "linear_up" "concave_up" ...
#>  $ n                 : int  100 100 100 100 100 ...
#>  $ sigma             : num  0.05 0.05 0.05 0.05 0.05 ...
#>  $ seed              : int  2027 2028 2029 2030 2031 ...
#>  $ predicted         : chr  "linear_up" "concave_up" ...
#>  $ correct           : logi  TRUE TRUE TRUE ...
#>  $ archetype_truth   : chr  "monotone_linear" "monotone_curved" ...
#>  $ archetype_pred    : chr  "monotone_linear" "monotone_curved" ...
#>  $ archetype_correct : logi  TRUE TRUE TRUE ...
#>  $ monotonicity_index: num  1 1 ...
#>  $ convexity_index   : num  0 -0.847 ...
#>  $ n_turn            : int  0 0 1 1 1 ...
#>  $ n_inflect         : int  0 0 0 0 2 ...
#>  $ error             : chr  NA NA ...

Recovery curves (headline figure)

janusplot_shape_sensitivity_plot(shape_sensitivity_demo,
                                 "recovery_curves")

Line plot of archetype-recovery accuracy versus sigma, one line per shape, faceted or coloured by sample size; five lines stay high across the noise range and one (wave) is flat at zero.

Archetype-recovery accuracy against noise level (sigma) for each of the six ground-truth shapes, at each of the three sample sizes.

Five of the six shapes are recovered well across the whole noise range, including at sigma = 0.40 (see H4 above): bimodal, concave_up and inverted_u hold at 1.00 throughout at n = 500, and u_shape only drops to 0.97 at the highest noise level. linear_up is the outlier among the five – it never exceeds 0.73 even at sigma = 0.05, so its curve is flat and mediocre rather than a high-noise-tolerance decay. The sixth shape, wave, is flat at 0.00 across every n and sigma cell: this is the generator defect described under H1, not a recovered-then-lost noise-tolerance curve, and should not be read alongside the other five.

Archetype confusion

janusplot_shape_sensitivity_plot(shape_sensitivity_demo,
                                 "confusion_archetype")

Confusion matrix of true archetype (rows) against predicted archetype (columns), pooled across all n, sigma and replicates in the demo.

Two off-diagonal patterns dominate here. wave is predicted unimodal on every single one of its 360 replicates – the direct consequence of the generator defect flagged under H1, since a curve with only one interior turning point classifies as unimodal by construction, regardless of noise. linear_up, by contrast, is a genuine noise-driven confusion: it lands in monotone_curved roughly 30% of the time, because sampling noise on a short curve occasionally tips the fitted monotonicity_index below the monotone-linear cutoff.

Archetype-level accuracy grid

janusplot_shape_sensitivity_plot(shape_sensitivity_demo,
                                 "accuracy_grid")

Grid of heatmap panels, one per ground-truth shape, with sample size on one axis and noise level on the other, cells shaded by archetype-recovery accuracy; five panels are uniformly dark (high accuracy) and one (wave) is uniformly pale (zero accuracy).

Heatmap of archetype-recovery accuracy across the full (n, sigma) design, one panel per shape.

Per-shape heatmap of P(archetype correct) across the (n, sigma) design. Reading across a row shows the noise-tolerance profile of one sample size; reading up a column shows the sample-size sensitivity at one noise level. The wave panel is uniformly at zero across the whole grid – flat cells here mean the generator defect above, not a genuinely noise-invariant failure mode.

Numerical summary

head(janusplot_shape_sensitivity_summary(shape_sensitivity_demo,
                                         level = "archetype"), 10)
#>         truth   n sigma  accuracy
#> 1     bimodal 100  0.05 1.0000000
#> 2  concave_up 100  0.05 1.0000000
#> 3  inverted_u 100  0.05 1.0000000
#> 4   linear_up 100  0.05 0.6666667
#> 5     u_shape 100  0.05 1.0000000
#> 6        wave 100  0.05 0.0000000
#> 7     bimodal 200  0.05 1.0000000
#> 8  concave_up 200  0.05 1.0000000
#> 9  inverted_u 200  0.05 1.0000000
#> 10  linear_up 200  0.05 0.7666667

Running your own sweep

The demo is a starting point. For the publication-grade figure use the full default grid (14 shapes × 4 sample sizes × 5 noise levels × 200 reps = 56 000 fits):

# Configure parallel execution (optional) -- you control the plan.
future::plan(future::multisession, workers = 4L)

res <- janusplot_shape_sensitivity(parallel = TRUE)

# Save for your paper
saveRDS(res, "shape_sensitivity_full.rds")
janusplot_shape_sensitivity_plot(res, "recovery_curves")

Custom shape subsets + cutoffs

Every argument is tunable. mono_strong and curv_low are only read on the monotone dispatch path (n_turning_points == 0 and n_inflections == 0), so a worked example needs a monotone shape family to actually exercise them – linear_up, concave_up and convex_up below, rather than the wave/multimodal family, where these two cutoffs are never consulted. Tightening mono_strong raises the bar for calling a curve “strictly monotone” (versus s_shape/rippled), and lowering curv_low raises the bar for calling it “near-linear” (versus curved).

strict <- janusplot_shape_cutoffs(mono_strong = 0.95, curv_low = 0.1)

res_strict <- janusplot_shape_sensitivity(
  shapes     = c("linear_up", "concave_up", "convex_up"),
  n_grid     = c(200L, 500L),
  sigma_grid = c(0.05, 0.10, 0.20),
  n_rep      = 100L,
  cutoffs    = strict
)

janusplot_shape_sensitivity_summary(res_strict, level = "fine")

References

Pya, N., & Wood, S. N. (2015). Shape constrained additive models. Statistics and Computing, 25(3), 543–559.
Calabrese, E. J. (2008). Hormesis: why it is important to toxicology and toxicologists. Environmental Toxicology and Chemistry, 27(7), 1451–1474.
Milnor, J. (1963). Morse Theory. Princeton University Press.
Meyer, M. C. (2008). Inference using shape-restricted regression splines. Annals of Applied Statistics, 2(3), 1013–1033.

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