Finding structure in Palmer penguins with sigPCA

Overview

Principal component analysis (PCA) orders axes by the amount of variation they explain, but it does not determine how many of those axes contain structure beyond sampling noise. A scree plot can suggest a cutoff, although that choice is often subjective. sigPCA instead compares the observed eigenvalues with a null spectrum.

The palmerpenguins PCA vignette develops a useful exploratory sequence: inspect the relationships among the morphological measurements, standardize them, fit a PCA, and interpret the scores and loadings. Friendly’s multivariate analysis emphasizes the biplot as a joint view of observations and variables. We follow that general sequence, but add a question between fitting and interpreting the PCA: which components depart from a noise-only spectrum?

We first use the Marchenko–Pastur bounds and then compare the result with a permutation test. Species labels are withheld from both tests and used only afterward to interpret the components.

Start from the morphological measurements

The penguins data contain four continuous measurements of body and bill dimensions. We retain those variables, remove rows with missing measurements, and keep the species label separately for later interpretation.

measurement_names <- c(
  "bill_length_mm",
  "bill_depth_mm",
  "flipper_length_mm",
  "body_mass_g"
)

penguins <- palmerpenguins::penguins
complete_rows <- stats::complete.cases(penguins[, measurement_names])

x <- as.matrix(penguins[complete_rows, measurement_names])

c(
  observations = nrow(x),
  measurements = ncol(x),
  omitted_rows = sum(!complete_rows)
)
#> observations measurements omitted_rows 
#>          342            4            2

The correlation matrix gives the first indication that the four measurements do not vary independently.

round(stats::cor(x), 2)
#>                   bill_length_mm bill_depth_mm flipper_length_mm body_mass_g
#> bill_length_mm              1.00         -0.24              0.66        0.60
#> bill_depth_mm              -0.24          1.00             -0.58       -0.47
#> flipper_length_mm           0.66         -0.58              1.00        0.87
#> body_mass_g                 0.60         -0.47              0.87        1.00

Flipper length and body mass form the strongest positive pair. Bill depth is negatively associated with the other measurements when the species are pooled. PCA summarizes these overlapping relationships as orthogonal axes rather than examining each pair in isolation.

The variables are measured on very different scales: millimetres for bill and flipper measurements, and grams for body mass. We therefore center and scale them so that the PCA is based on their correlation structure instead of being dominated by body mass merely because of its units.

Fit the conventional PCA

penguin_pca <- stats::prcomp(
  x,
  center = TRUE,
  scale. = TRUE
)

eigenvalues <- penguin_pca$sdev^2
variance_explained <- 100 * eigenvalues / sum(eigenvalues)

pca_summary <- data.frame(
  component = paste0("PC", seq_along(eigenvalues)),
  eigenvalue = round(eigenvalues, 3),
  variance_percent = round(variance_explained, 1),
  cumulative_percent = round(cumsum(variance_explained), 1)
)

knitr::kable(pca_summary)

The usual summary makes the first two components look attractive: together they explain about 88% of the standardized variance. Explained variance, however, is a descriptive quantity. It does not establish whether an observed eigenvalue is larger than expected under a null model.

Test the eigenvalue spectrum

The Marchenko–Pastur (MP) distribution describes the eigenvalue spectrum expected under a noise model. For a matrix with \(n\) observations and \(p\) standardized variables, sigPCA calculates the bounds

\[ \lambda_{\pm} = \left(1 \pm \sqrt{p/n}\right)^2. \]

Eigenvalues above the upper bound are candidates for structured variation that cannot be explained by this null model. We request both the MP and permutation analyses in one call.

set.seed(2026)

penguin_sig <- sigPCA::sigPCA(
  x,
  method = "both",
  num_permutations = 999
)

penguin_sig$mp$mp_bounds
#> $lambda_min
#> [1] 0.7954007
#> 
#> $lambda_max
#> [1] 1.227991
penguin_sig$combined
#> $significant_components
#> [1] 1
#> 
#> $consensus
#> [1] 1

The component-level results make the two comparisons explicit.

component_results <- data.frame(
  component = paste0("PC", seq_along(eigenvalues)),
  eigenvalue = round(eigenvalues, 3),
  variance_percent = round(variance_explained, 1),
  beyond_mp_upper = seq_along(eigenvalues) %in%
    penguin_sig$mp$significant_components,
  permutation_p = round(penguin_sig$perm$pvalues, 4)
)

knitr::kable(component_results)

PC1 lies above the MP upper bound and is also supported by the permutation analysis. PC2 explains a visible fraction of the variance, but it is not selected by the upper-tail MP rule. This is precisely the distinction that a conventional scree plot cannot make by itself.

sigPCA::plot_sigPCA(
  penguin_sig$mp$eigenvalues,
  penguin_sig$mp$mp_bounds
)

Observed eigenvalues and the Marchenko–Pastur bounds. Components above the upper bound are highlighted.

The component-by-component plot makes the retention decision easy to inspect. The package also provides plot_sigPCA_histogram(), based on an original design by Enrique Hernández-Lemus, as a complementary view of the empirical eigenvalue distribution. Dashed lines mark both MP bounds, while rug marks identify eigenvalues above the upper bound. Because this example contains only four eigenvalues, we use one histogram bin per component; the distributional view becomes more informative as the number of variables increases.

sigPCA::plot_sigPCA_histogram(
  penguin_sig,
  bins = ncol(x)
)

Empirical eigenvalue distribution with the Marchenko–Pastur bounds. Rug marks identify eigenvalues above the upper bound.

The two methods use related but different null models. The MP test compares the observed spectrum with a theoretical spectrum determined by \(p/n\). The permutation test independently shuffles each variable, preserving its marginal distribution while destroying associations among variables. Agreement between them strengthens the conclusion that the leading eigenvalue reflects multivariate structure.

Interpret the retained structure

sigPCA addresses component retention; the ordinary PCA outputs address interpretation. The loadings below show which measurements define PC1. Their signs may reverse without changing the PCA solution, so direction matters only relative to the other loadings.

loading_data <- data.frame(
  measurement = rownames(penguin_pca$rotation),
  loading = penguin_pca$rotation[, 1]
)

ggplot2::ggplot(loading_data) +
  ggplot2::aes(
    x = abs(loading),
    y = stats::reorder(measurement, abs(loading)),
    fill = loading > 0
  ) +
  ggplot2::geom_col() +
  ggplot2::scale_fill_manual(
    values = c("#b6dfe2", "#0A537D"),
    labels = c("Negative", "Positive")
  ) +
  ggplot2::labs(
    x = "Absolute loading",
    y = NULL,
    fill = "Sign"
  ) +
  ggplot2::theme_minimal()

Absolute loadings for the retained first component. Fill indicates the loading sign.

Flipper length and body mass contribute strongly and in the same direction, consistent with their correlation. Bill length points in that direction as well, whereas bill depth opposes the other measurements. PC1 therefore captures a broad morphological contrast rather than reproducing any single measurement.

A biplot places that axis back in its phenomenological context. Species is used here for colour and shape, but it played no role in fitting the PCA or testing its eigenvalues. The loading vectors are multiplied by a common factor so they can be displayed on the score scale without changing their relative angles or lengths.

score_data <- data.frame(
  PC1 = penguin_pca$x[, 1],
  PC2 = penguin_pca$x[, 2],
  species = penguins$species[complete_rows]
)

loading_vectors <- data.frame(
  measurement = rownames(penguin_pca$rotation),
  PC1 = penguin_pca$rotation[, 1],
  PC2 = penguin_pca$rotation[, 2]
)

score_span <- vapply(
  score_data[, c("PC1", "PC2")],
  function(z) diff(range(z)),
  numeric(1)
)

loading_span <- vapply(
  loading_vectors[, c("PC1", "PC2")],
  function(z) diff(range(z)),
  numeric(1)
)

arrow_multiplier <- 0.35 * min(score_span / loading_span)
loading_vectors$PC1 <- loading_vectors$PC1 * arrow_multiplier
loading_vectors$PC2 <- loading_vectors$PC2 * arrow_multiplier

penguin_colours <- c(
  Adelie = "darkorange",
  Chinstrap = "purple",
  Gentoo = "cyan4"
)

ggplot2::ggplot(score_data) +
  ggplot2::aes(x = PC1, y = PC2, colour = species, shape = species) +
  ggplot2::geom_point(alpha = 0.75, size = 2) +
  ggplot2::geom_segment(
    data = loading_vectors,
    ggplot2::aes(x = 0, y = 0, xend = PC1, yend = PC2),
    inherit.aes = FALSE,
    colour = "#0A537D",
    arrow = grid::arrow(length = grid::unit(0.08, "inches"))
  ) +
  ggplot2::geom_text(
    data = loading_vectors,
    ggplot2::aes(x = PC1, y = PC2, label = measurement),
    inherit.aes = FALSE,
    colour = "#0A537D",
    hjust = 0,
    vjust = 1,
    size = 3.5,
    check_overlap = TRUE
  ) +
  ggplot2::scale_colour_manual(values = penguin_colours) +
  ggplot2::labs(
    x = sprintf("PC1 (%.1f%% of variance)", variance_explained[1]),
    y = sprintf("PC2 (%.1f%% of variance)", variance_explained[2]),
    colour = "Species",
    shape = "Species"
  ) +
  ggplot2::theme_minimal()

Scores and loading vectors for the first two components. Species labels are used only for interpretation.

The retained first axis separates much of the Gentoo morphology from Adelie and Chinstrap morphology. PC2 helps display the contrast in bill dimensions and the separation between Adelie and Chinstrap penguins, as in earlier exploratory analyses of these data, but it is not selected by the upper-tail MP criterion. A component can therefore remain useful as a descriptive coordinate without being retained as evidence of structure under a particular null model.

What about eigenvalues below the lower bound?

Several eigenvalues fall below the MP lower bound. They should not be interpreted as additional high-variance components to retain. A small eigenvalue indicates a direction in which the data vary less than the null model predicts, often because correlated variables create redundancy or an approximately constrained combination of measurements. Thus, departures on the two sides have different meanings:

• eigenvalues above the upper bound identify unusually strong axes of variation;

• eigenvalues below the lower bound identify unusually compressed directions.

The current sigPCA retention rule reports the first case through significant_components. The lower tail can still be scientifically interesting when the goal is to study constraints, collinearity, or redundant measurements.

Practical considerations

The MP comparison is a null-model diagnostic, not a guarantee that a component is important in its phenomenological context. Its interpretation depends on preprocessing and on the suitability of the random-matrix null model. This example has many more observations than variables and is intended to make the workflow transparent; MP results are especially useful in larger, higher-dimensional applications.

Missing values must be handled before calling sigPCA. Complete-case removal is adequate for this demonstration, but it may introduce bias in other data. Likewise, scaling is appropriate here because the measurements use different units. It need not be appropriate when absolute variance carries the scientific meaning of interest.

References

Friendly, M. (2020). Penguin data: Multivariate EDA. RPubs. https://rpubs.com/friendly/penguin-biplots

Horst, A. M., Hill, A. P., and Gorman, K. B. PCA with penguins and recipes. palmerpenguins package vignette. https://allisonhorst.github.io/palmerpenguins/articles/pca.html

Horst, A. M., Hill, A. P., and Gorman, K. B. (2022). Palmer Archipelago Penguins Data in the palmerpenguins R Package: An Alternative to Anderson’s Irises. The R Journal, 14(1), 244–254. https://doi.org/10.32614/RJ-2022-020

Marchenko, V. A. and Pastur, L. A. (1967). Distribution of eigenvalues for some sets of random matrices. Mathematics of the USSR-Sbornik, 1(4), 457–483.