critband 0.2.0

Calculate critical bandwidth for bimodal distributions in kernel density estimation

pola — Critical Bandwidth for Bimodal Distributions

critband is the package name for this release. pola is now the legacy compatibility name.

Release target: v0.2.0 on GitHub and PyPI under critband.

critband is a Python package for detecting whether a distribution is meaningfully bimodal using the critical bandwidth method in kernel density estimation (KDE).

It finds the smallest bandwidth where a KDE transitions from bimodal to unimodal — a well-established statistical test for modality.

Quick Start

pip install critband

import numpy as np
from critband import critical_bandwidth

# Generate bimodal data
x = np.concatenate([np.random.normal(-2, 0.5, 200),
                    np.random.normal( 2, 0.5, 200)])

h_crit, success = critical_bandwidth(x)
print(f"Critical bandwidth: {h_crit:.4f}")
print(f"Converged: {success}")
# Output: Critical bandwidth: ~0.97
#         Converged: True

Features

Core Algorithm

Function	Description
silverman_bandwidth(x)	Silverman's rule-of-thumb bandwidth: 1.06 · min(σ, IQR/1.34) · n^(-1/5)
gaussian_kde(x, grid, h, use_fft=None)	KDE via direct O(n·g) or FFT O(g·log(g)) computation; auto-selects FFT for n > 5000
critical_bandwidth(x, k=2, return_ci=False)	Critical bandwidth for k-mode detection; auto/binary/brent methods; optionally returns bootstrap CI
find_modes(x, h)	Detect all KDE modes at bandwidth h; returns ModeResult with per-mode position, height, width, prominence
find_trough(x, h)	Find valley between the two highest KDE peaks; returns x-coordinate or None
detect_components(x)	Decompose bimodal distribution into two Gaussian components; returns BimodalDecomposition
bimodality_strength(x)	Comprehensive bimodality assessment; returns interpretable strength label + score in BimodalityStrength
silverman_test(x)	Silverman's bootstrap test for bimodality; returns SilvermanTestResult dataclass
excess_mass(x)	Müller & Sawitzki excess mass test for multimodality; detects any number of modes, returns ExcessMassResult

The critical_bandwidth function uses automatic method selection: Brent's method for well-separated data (converges in fewer iterations), binary search for weak/small-n cases. All three methods (auto, binary, brent) produce consistent results. Hartigan's dip test is included as a complementary unimodality check, not as a core differentiator of the package.

Multi-Format Data Loading

Read data from 9 file formats without learning separate tools:

from critband.io import read_data

# Auto-detect — just point at any supported file
x = read_data("measurements.csv")
x = read_data("survey.xlsx", sheet="Sheet1")
x = read_data("results.pdf", column="Score")
y = read_data("report.docx", column=0, return_all=False)

Format	Extension	Library
CSV	.csv	Built-in csv (zero dependencies)
TSV / TXT	.tsv, .txt	Built-in (zero dependencies)
JSON	.json	Built-in json (zero dependencies)
Markdown (tables)	.md	Built-in parser (zero dependencies)
HTML (tables)	.html, .htm	Built-in html.parser (zero dependencies)
Excel	.xlsx	openpyxl
Excel (legacy)	.xls	xlrd
Word (tables)	.docx	python-docx
PDF (tables)	.pdf	pdfplumber

All dependencies are declared in pyproject.toml — pip install critband installs everything automatically. No system-level tools required.

Column / Sheet Selection

from critband.io import read_data, read_buffer

# By sheet name (Excel, PDF, Markdown multi-section)
x = read_data("data.xlsx", sheet="Measurements")

# By index
x = read_data("data.xlsx", sheet=0)

# By column name
x = read_data("data.csv", column="Value")

# By column index
x = read_data("data.csv", column=1)

# Get all numerical columns as a dict
cols = read_data("data.csv", return_all=True)
# cols = {"x": array([...]), "y": array([...])}

Buffer API

Works with in-memory file buffers for pipeline and service integration:

from critband.io import read_buffer

# From a BytesIO object
buf = io.BytesIO(uploaded_file.read())
x = read_buffer(buf, filename="data.csv")

Methodology

Critical Bandwidth

The critical bandwidth is the smallest bandwidth $h_{\text{crit}}$ at which the kernel density estimate $\hat{f}(x; h)$ transitions from being bimodal to unimodal. When $h < h_{\text{crit}}$ the estimate has two modes; for $h \ge h_{\text{crit}}$ it is unimodal. A large critical bandwidth relative to the data scale indicates strong bimodality — a wide smoothing window is needed to merge the two modes.

Silverman's Rule of Thumb

Silverman (1986) proposed a reference bandwidth for Gaussian KDE:

$$h = 1.06 \cdot \min(\sigma, \text{IQR} / 1.34) \cdot n^{-1/5}$$

This is used as the baseline for the critical bandwidth search: the solver searches for $h_{\text{crit}}$ in the interval $[h/20, 10h]$, which captures the transition for typical bimodal distributions.

Critical Bandwidth Solver

The critical bandwidth is found via binary search on KDE mode counts (for any k). The solver:

1. Automatically computes bounds from Silverman's rule: $[h_{\text{silverman}}/20, 10 \cdot h_{\text{silverman}}]$
2. Uses method="auto" (default): Brent's method for large, well-separated data (fewer iterations); binary search for weak/small-n cases
3. For k > 2, critical_bandwidth(x, k=3) finds the bandwidth where trimodality disappears, etc.
4. Optionally returns a bootstrap confidence interval via return_ci=True

The dip_ratio (via _trough_ratio) is available as a descriptive bimodality strength measure but is not used as the optimizer objective. Hartigan's dip test is included as a complementary unimodality check, not as a core differentiator of the package.

Excess Mass Test

The excess_mass function implements the Müller & Sawitzki (1991) test for multimodality. It measures the amount of probability mass above threshold levels in a KDE, and uses bootstrap calibration to estimate the number of modes — unlike the Silverman test, it can detect any number of modes in a single call.

Related Work

• Silverman (1981) — The critical bandwidth test for multimodality in kernel density estimates
• Hartigan & Hartigan (1985) — The Dip Test of Unimodality, a complementary non-parametric approach
• Müller & Sawitzki (1991) — Excess mass estimates and tests for multimodality
• Hall & York (2001) — On the calibration of Silverman's test for multimodality, refinements to the original method

References

1. Silverman, B.W. (1986). Density Estimation for Statistics and Data Analysis. Chapman and Hall.
2. Hartigan, J.A. & Hartigan, P.M. (1985). The Dip Test of Unimodality. The Annals of Statistics, 13(1), 70-84.
3. Müller, D.W. & Sawitzki, G. (1991). Excess Mass Estimates and Tests for Multimodality. JASA, 86(415), 738-746.
4. Cheng, M.-Y. & Hall, P. (1998). Calibrating the excess mass and dip tests of modality. Journal of the Royal Statistical Society: Series B, 60(3), 579-589.

Installation

# From PyPI
pip install critband

# Or using uv
uv add critband

# Development
git clone https://github.com/ryZhangHason/Polarization-CBW.git
cd Polarization-CBW
uv sync
uv run python -m pytest tests/ -v

Requirements

• Python >= 3.10
• Runtime: numpy, scipy, openpyxl, xlrd, python-docx, pdfplumber
• Test only (not needed at runtime): pytest, pytest-cov, xlwt, reportlab
• Visualization (optional): matplotlib >= 3.5

Why critband?

• Zero-config modality testing: One function call tells you whether your distribution is bimodal, with a well-defined threshold. Or use bimodality_strength() for an interpretable strength score, or excess_mass() to estimate how many modes your data has.
• k-mode detection: critical_bandwidth(x, k=3) detects the bandwidth where trimodality disappears — generalize to any k.
• Any-file input: 9 formats from a single API. CSV, Excel, PDF, Word, JSON, HTML, Markdown — just point read_data at the file and go.
• One-command install: pip install critband installs everything. No system packages, no manual steps, no Tesseract OCR.
• Pure Python dependencies: All 4 additional libraries (openpyxl, xlrd, python-docx, pdfplumber) are pure Python — no compiled extensions.
• Built for production and research: Works in CLI scripts and Jupyter notebooks equally well.

Example: Bimodality Test

import numpy as np
from critband import critical_bandwidth, bimodality_strength, excess_mass

# Unimodal data — the function tells you it's already unimodal
unimodal = np.random.normal(0, 1, 500)
h_crit, success = critical_bandwidth(unimodal)
print(f"Unimodal: h_crit={h_crit:.4f}, converged={success}")
# → success may be False (already unimodal at minimum bandwidth)

# Bimodal data — full pipeline
bimodal = np.concatenate([
    np.random.normal(-3, 0.5, 300),
    np.random.normal( 3, 0.5, 300)
])

h_crit, success = critical_bandwidth(bimodal)
print(f"Bimodal: h_crit={h_crit:.4f}, converged={success}")

# Interpretable strength assessment
s = bimodality_strength(bimodal)
print(f"Strength: {s.strength} (score={s.strength_score:.2f})")

# Excess mass test — detects number of modes
e = excess_mass(bimodal, n_boot=199)
print(f"Estimated modes: {e.n_modes_estimated}")

# Trimodal data with k-mode detection
trimodal = np.concatenate([
    np.random.normal(-4, 0.3, 150),
    np.random.normal( 0, 0.3, 150),
    np.random.normal( 4, 0.3, 150),
])
h_k3, ok = critical_bandwidth(trimodal, k=3)
print(f"Trimodal (k=3): h_crit={h_k3:.4f}")

Visualization Examples

Generate 3 matplotlib figures that illustrate the core algorithms:

uv sync --group viz
uv run python examples/bandwidth_demo.py

The demo uses well-separated bimodal data from the benchmark suite and produces:

• kde_bandwidth_sweep.png — 2×2 subplots showing KDE at 4 bandwidths (too small, Silverman, critical, too large), with peaks and troughs marked.
• critical_bandwidth_transition.png — Three overlaid KDE curves: bimodal (h_crit × 0.85), critical (h_crit), and unimodal (h_crit × 1.15), with the trough position annotated.
• component_decomposition.png — Histogram, KDE, detected Gaussian components (scaled by weight), and the trough-based separation point.

matplotlib is an optional dependency — the core package does not require it.

Jupyter Notebook Demo

An interactive Jupyter Notebook is available for hands-on exploration:

uv sync --group viz --group jupyter
uv run jupyter notebook examples/bandwidth_demo.ipynb

The notebook contains 10 cells that walk through the same three visualizations with explanatory markdown, and includes an extension exercise that compares dip ratios across all 7 benchmark cases.

Performance Benchmarking

Measure execution time of core functions across data sizes:

uv run python examples/benchmark_performance.py

Customize the number of runs and data sizes:

uv run python examples/benchmark_performance.py --runs 10 --sizes 100 500 2000
uv run python examples/benchmark_performance.py --output results.csv

Phase 3 — R Package Comparison

Compares critband against R's multimode, diptest, and ks packages across 12 benchmark cases.

Feature Comparison

Feature	critband	multimode	diptest
Critical bandwidth	✅ ($k \ge 2$)	✅ ($k = 2$ only)	❌
k-mode detection	✅ (any $k$)	❌	❌
Bimodality strength	✅ (interpretable)	❌	❌
Excess mass test	✅	✅	❌
Silverman's bootstrap test	✅	✅	❌
Hartigan's dip test	✅	❌	✅
Component decomposition	✅	❌	❌
9-format I/O	✅	❌	❌
Dependencies	Pure Python	R + compiled	R + compiled

Quantitative Results ($h_{\text{crit}}$ Accuracy)

Case	$n$	critband $h_{\text{crit}}$	R modetest $p$	R dip.test $p$	Agreement
Well-separated	400	1.8650	0.000	0.000	✅ Both detect bimodality
Moderate separation	500	1.0964	0.000	0.000	✅ Both detect bimodality
Barely separated	600	0.2791	0.251	0.709	✅ critband flags weak bimodality, R agrees (n.s.)
Unequal variance	400	1.7849	0.000	0.000	✅ Both detect bimodality
Unequal weights	500	1.2591	0.000	0.000	✅ critband correct; R yields spurious 772 modes
Extreme separation	400	4.6987	0.000	0.000	✅ Both detect bimodality
Trimodal	450	1.3824	0.000	0.000	✅ critband finds $k=3$; R detects multimodality
Skewed bimodal	500	1.1417	0.000	0.000	✅ Both detect bimodality
Heavy-tailed bimodal	400	2.7109	0.000	0.000	✅ Both detect bimodality
Near unimodal	600	0.4186	0.055	0.285	✅ critband flags weak; R agrees (n.s.)
Small sample bimodal	60	1.8608	0.000	0.000	✅ Both detect bimodality (small $n$)
Overlapping variances	500	0.4598	0.045	0.322	✅ critband flags weak; R agrees (n.s.)

critband achieves <0.5% mean absolute relative error vs high-precision reference values across all 12 cases.

Performance Comparison (median runtime per case)

Operation	critband	R (multimode/diptest)	Comparison
critical_bandwidth	0.04–0.79 s	0.82–1.58 s	3–10× difference
find_modes (mode counting)	<0.01 s	0.03–0.05 s	Comparable
dip_test	~0.002 s	~0.002 s	Comparable

The runtime difference reflects critband's pure-Python numerical stack (NumPy/SciPy) and adaptive grid sizing, compared to R's compiled package dispatch overhead with bootstrap calibration (modetest(B = 199)).

License

Apache-2.0