Weight Functions¶

Kernel functions for distance weighting.

Overview¶

Weight functions (kernels) determine how neighboring points contribute to each local fit. Points closer to the target receive higher weights.

Weight Functions

Available Kernels¶

Kernel	Efficiency	Smoothness	Support
Tricube	0.998	Very smooth	Compact
Epanechnikov	1.000	Smooth	Compact
Gaussian	0.961	Infinite	Unbounded
Biweight	0.995	Very smooth	Compact
Cosine	0.999	Smooth	Compact
Triangle	0.989	Moderate	Compact
Uniform	0.943	None	Compact

Efficiency = AMISE relative to Epanechnikov (1.0 = optimal)

Tricube (Default)¶

Cleveland's original choice. Best all-around performance.

\[w(u) = (1 - |u|^3)^3\]

Use when: Default choice for most applications.

RPythonRustJuliaNode.jsWebAssemblyC++

model <- Loess(weight_function = "tricube")
result <- model$fit(x, y)

model = fl.Loess(weight_function="tricube")
result = model.fit(x, y)

let model = Loess::new()
    .weight_function("tricube")
    .build()?;
let result = model.fit(&x, &y)?;

model = Loess(; weight_function="tricube")
result = fit(model, x, y)

const model = new Loess({ weight_function: "tricube" });
const result = model.fit(x, y);

const model = new Loess({ weight_function: "tricube" });
const result = model.fit(x, y);

fastloess::Loess model({ .weight_function = "tricube" });
auto result = model.fit(x, y).value();

Epanechnikov¶

Theoretically optimal for kernel density estimation.

\[w(u) = \frac{3}{4}(1 - u^2)\]

Use when: Optimal MSE properties desired.

RPythonRustJuliaNode.jsWebAssemblyC++

model <- Loess(weight_function = "epanechnikov")
result <- model$fit(x, y)

model = fl.Loess(weight_function="epanechnikov")
result = model.fit(x, y)

let model = Loess::new()
    .weight_function("epanechnikov")
    .build()?;
let result = model.fit(&x, &y)?;

model = Loess(; weight_function="epanechnikov")
result = fit(model, x, y)

const model = new Loess({ weight_function: "epanechnikov" });
const result = model.fit(x, y);

const model = new Loess({ weight_function: "epanechnikov" });
const result = model.fit(x, y);

fastloess::Loess model({ .weight_function = "epanechnikov" });
auto result = model.fit(x, y).value();

Gaussian¶

Infinitely smooth. No boundary effects.

\[w(u) = \exp(-u^2/2)\]

Use when: Maximum smoothness needed, computational cost acceptable.

RPythonRustJuliaNode.jsWebAssemblyC++

model <- Loess(weight_function = "gaussian")
result <- model$fit(x, y)

model = fl.Loess(weight_function="gaussian")
result = model.fit(x, y)

let model = Loess::new()
    .weight_function("gaussian")
    .build()?;
let result = model.fit(&x, &y)?;

model = Loess(; weight_function="gaussian")
result = fit(model, x, y)

const model = new Loess({ weight_function: "gaussian" });
const result = model.fit(x, y);

const model = new Loess({ weight_function: "gaussian" });
const result = model.fit(x, y);

fastloess::Loess model({ .weight_function = "gaussian" });
auto result = model.fit(x, y).value();

Biweight¶

Good balance of efficiency and smoothness.

\[w(u) = (1 - u^2)^2\]

Use when: Alternative to Tricube with slightly different properties.

RPythonRustJuliaNode.jsWebAssemblyC++

model <- Loess(weight_function = "biweight")
result <- model$fit(x, y)

model = fl.Loess(weight_function="biweight")
result = model.fit(x, y)

let model = Loess::new()
    .weight_function("biweight")
    .build()?;
let result = model.fit(&x, &y)?;

model = Loess(; weight_function="biweight")
result = fit(model, x, y)

const model = new Loess({ weight_function: "biweight" });
const result = model.fit(x, y);

const model = new Loess({ weight_function: "biweight" });
const result = model.fit(x, y);

fastloess::Loess model({ .weight_function = "biweight" });
auto result = model.fit(x, y).value();

Cosine¶

Smooth and computationally efficient.

\[w(u) = \cos(\pi u / 2)\]

Use when: Want smooth kernel with simple form.

RPythonRustJuliaNode.jsWebAssemblyC++

model <- Loess(weight_function = "cosine")
result <- model$fit(x, y)

model = fl.Loess(weight_function="cosine")
result = model.fit(x, y)

let model = Loess::new()
    .weight_function("cosine")
    .build()?;
let result = model.fit(&x, &y)?;

model = Loess(; weight_function="cosine")
result = fit(model, x, y)

const model = new Loess({ weight_function: "cosine" });
const result = model.fit(x, y);

const model = new Loess({ weight_function: "cosine" });
const result = model.fit(x, y);

fastloess::Loess model({ .weight_function = "cosine" });
auto result = model.fit(x, y).value();

Triangle¶

Simple linear taper.

\[w(u) = 1 - |u|\]

Use when: Simple, interpretable weights.

RPythonRustJuliaNode.jsWebAssemblyC++

model <- Loess(weight_function = "triangle")
result <- model$fit(x, y)

model = fl.Loess(weight_function="triangle")
result = model.fit(x, y)

let model = Loess::new()
    .weight_function("triangle")
    .build()?;
let result = model.fit(&x, &y)?;

model = Loess(; weight_function="triangle")
result = fit(model, x, y)

const model = new Loess({ weight_function: "triangle" });
const result = model.fit(x, y);

const model = new Loess({ weight_function: "triangle" });
const result = model.fit(x, y);

fastloess::Loess model({ .weight_function = "triangle" });
auto result = model.fit(x, y).value();

Uniform¶

Equal weights within window. Fastest but least smooth.

\[w(u) = 1\]

Use when: Speed is critical, smoothness less important.

RPythonRustJuliaNode.jsWebAssemblyC++

model <- Loess(weight_function = "uniform")
result <- model$fit(x, y)

model = fl.Loess(weight_function="uniform")
result = model.fit(x, y)

let model = Loess::new()
    .weight_function("uniform")
    .build()?;
let result = model.fit(&x, &y)?;

model = Loess(; weight_function="uniform")
result = fit(model, x, y)

const model = new Loess({ weight_function: "uniform" });
const result = model.fit(x, y);

const model = new Loess({ weight_function: "uniform" });
const result = model.fit(x, y);

fastloess::Loess model({ .weight_function = "uniform" });
auto result = model.fit(x, y).value();

Choosing a Kernel¶

flowchart TD A[Choose Kernel] --> B{Need maximum smooth} B -- Yes --> C[Gaussian] B -- No --> D{Default acceptable} D -- Yes --> E[Tricube] D -- No --> F{Optimal MSE} F -- Yes --> G[Epanechnikov] F -- No --> H{Speed critical} H -- Yes --> I[Uniform] H -- No --> J[Biweight]

Recommendation

Stick with Tricube (default) unless you have specific requirements. The differences between kernels are usually small in practice.