# Volatility Processes¶

A volatility process is added to a mean model to capture time-varying volatility.

 ConstantVariance() Constant volatility process GARCH([p, o, q, power]) GARCH and related model estimation FIGARCH([p, q, power, truncation]) FIGARCH model EGARCH([p, o, q]) EGARCH model estimation HARCH([lags]) Heterogeneous ARCH process MIDASHyperbolic([m, asym]) MIDAS Hyperbolic ARCH process ARCH([p]) ARCH process

## Parameterless Variance Processes¶

Some volatility processes use fixed parameters and so have no parameters that are estimable.

 EWMAVariance([lam]) Exponentially Weighted Moving-Average (RiskMetrics) Variance process RiskMetrics2006([tau0, tau1, kmax, rho]) RiskMetrics 2006 Variance process

## FixedVariance¶

The FixedVariance class is a special-purpose volatility process that allows the so-called zig-zag algorithm to be used. See the example for usage.

 FixedVariance(variance[, unit_scale]) Fixed volatility process

## Writing New Volatility Processes¶

All volatility processes must inherit from :class:VolatilityProcess and provide all public methods.

 VolatilityProcess() Abstract base class for ARCH models.