# Univariate

An instance of UnivariateARCHModel contains a vector of data (such as equity returns), and encapsulates information about the volatility specification (e.g., GARCH or EGARCH), the mean specification (e.g., whether an intercept is included), and the error distribution.

In general a univariate model can be written

$$$r_t = \mu_t + \sigma_t z_t, \quad z_t \stackrel{\text{iid}}{\sim} F.$$$

Hence, a univariate model is a triple of functions $\left(\mu_t, \sigma_t, F \right)$. The table below lists current options for the conditional mean, conditional variance, and the error distribution.

$\mu_t$$\sigma_t$$F$
NoInterceptARCH{0} (constant)StdNormal
InterceptARCH{q}StdT
ARMA{p,q}GARCH{p,q}StdGED
Regression(X)TGARCH{o,p,q}Std User-Defined
EGARCH{o,p,q}

Details on these options are given below.

## Volatility specifications

Volatility specifications describe the evolution of $\sigma_t$. They are modelled as subtypes of UnivariateVolatilitySpec. There is one type for each class of (G)ARCH model, parameterized by the number(s) of lags (e.g., $p$, $q$ for a GARCH(p, q) model). For each volatility specification, the order of the parameters in the coefficient vector is such that all parameters pertaining to the first type parameter ($p$) appear before those pertaining to the second ($q$).

### ARCH

With $a_t\equiv r_t-\mu_t$, the ARCH(q) volatility specification, due to Engle (1982), is

$$$\sigma_t^2=\omega+\sum_{i=1}^q\alpha_i a_{t-i}^2, \quad \omega, \alpha_i>0,\quad \sum_{i=1}^{q} \alpha_i<1.$$$

The corresponding type is ARCH{q}. For example, an ARCH(2) model with $ω=1$, $α₁=.5$, and $α₂=.4$ is obtained with

julia> using ARCHModels

julia> ARCH{2}([1., .5, .4])
TGARCH{0, 0, 2} specification.

──────────────────────────
ω   α₁   α₂
──────────────────────────
Parameters:  1.0  0.5  0.4
──────────────────────────

### GARCH

The GARCH(p, q) model, due to Bollerslev (1986), specifies the volatility as

$$$\sigma_t^2=\omega+\sum_{i=1}^p\beta_i \sigma_{t-i}^2+\sum_{i=1}^q\alpha_i a_{t-i}^2, \quad \omega, \alpha_i, \beta_i>0,\quad \sum_{i=1}^{\max p,q} \alpha_i+\beta_i<1.$$$

It is available as GARCH{p, q}:

julia> GARCH{1, 1}([1., .9, .05])
GARCH{1, 1} specification.

───────────────────────────
ω   β₁    α₁
───────────────────────────
Parameters:  1.0  0.9  0.05
───────────────────────────

This creates a GARCH(1, 1) specification with $ω=1$, $β=.9$, and $α=.05$.

### TGARCH

As may have been guessed from the output above, the ARCH and GARCH models are actually special cases of a more general class of models, known as TGARCH (Threshold GARCH), due to Glosten, Jagannathan, and Runkle (1993). The TGARCH{o, p, q} model takes the form

$$$\sigma_t^2=\omega+\sum_{i=1}^o\gamma_i a_{t-i}^2 1_{a_{t-i}<0}+\sum_{i=1}^p\beta_i \sigma_{t-i}^2+\sum_{i=1}^q\alpha_i a_{t-i}^2, \quad \omega, \alpha_i, \beta_i, \gamma_i>0, \sum_{i=1}^{\max o,p,q} \alpha_i+\beta_i+\gamma_i/2<1.$$$

The TGARCH model allows the volatility to react differently (typically more strongly) to negative shocks, a feature known as the (statistical) leverage effect. Is available as TGARCH{o, p, q}:

julia> TGARCH{1, 1, 1}([1., .04, .9, .01])
TGARCH{1, 1, 1} specification.

─────────────────────────────────
ω    γ₁   β₁    α₁
─────────────────────────────────
Parameters:  1.0  0.04  0.9  0.01
─────────────────────────────────

### EGARCH

The EGARCH{o, p, q} volatility specification, due to Nelson (1991), is

$$$\log(\sigma_t^2)=\omega+\sum_{i=1}^o\gamma_i z_{t-i}+\sum_{i=1}^p\beta_i \log(\sigma_{t-i}^2)+\sum_{i=1}^q\alpha_i (|z_{t-i}|-\sqrt{2/\pi}), \quad z_t=r_t/\sigma_t,\quad \sum_{i=1}^{p}\beta_i<1.$$$

Like the TGARCH model, it can account for the leverage effect. The corresponding type is EGARCH{o, p, q}:

julia> EGARCH{1, 1, 1}([-0.1, .1, .9, .04])
EGARCH{1, 1, 1} specification.

─────────────────────────────────
ω   γ₁   β₁    α₁
─────────────────────────────────
Parameters:  -0.1  0.1  0.9  0.04
─────────────────────────────────

## Mean specifications

Mean specifications serve to specify $\mu_t$. They are modelled as subtypes of MeanSpec. They contain their parameters as (possibly empty) vectors, but convenience constructors are provided where appropriate. The following specifications are available:

julia> NoIntercept() # convenience constructor, eltype defaults to Float64
NoIntercept{Float64}(Float64[])
julia> Intercept(3) # convenience constructor
Intercept{Float64}([3.0])
• A linear regression model: $\mu_t=\mathbf{x}_t^{\mathrm{\scriptscriptstyle T}}\boldsymbol{\beta}$. Available as Regression:
julia> X = ones(100, 1);

julia> reg = Regression(X);

In this example, we created a regression model containing one regressor, given by a column of ones; this is equivalent to including an intercept in the model (see Intercept above). In general, the constructor should be passed a design matrix $\mathbf{X}$ containing $\{\mathbf{x}_t^{\mathrm{\scriptscriptstyle T}}\}_{t=1\ldots T}$ as its rows; that is, for a model with $T$ observations and $k$ regressors, $X$ would have dimensions $T\times k$.

Another way to create a linear regression with ARCH errors is to pass a LinearModel or DataFrameRegressionModel from GLM.jl to fit, as described under Integration with GLM.jl.

• An ARMA(p, q) model: $\mu_t=c+\sum_{i=1}^p \varphi_i r_{t-i}+\sum_{i=1}^q \theta_i a_{t-i}$. Available as ARMA{p, q}:
julia> ARMA{1, 1}([1., .9, -.1])
ARMA{1, 1, Float64}([1.0, 0.9, -0.1])

Pure AR(p) and MA(q) models are obtained as follows:

julia> AR{1}([1., .9])
AR{1, Float64}([1.0, 0.9])
julia> MA{1}([1., -.1])
MA{1, Float64}([1.0, -0.1])

## Distributions

### Built-in distributions

Different standardized (mean zero, variance one) distributions for $z_t$ are available as subtypes of StandardizedDistribution. StandardizedDistribution in turn subtypes Distribution{Univariate, Continuous} from Distributions.jl, though not the entire interface need necessarily be implemented. StandardizedDistributions again hold their parameters as vectors, but convenience constructors are provided. The following are currently available:

julia> StdNormal() # convenience constructor
StdNormal{Float64}(coefs=Float64[])
julia> StdT(3) # convenience constructor
StdT{Float64}(coefs=[3.0])
julia> StdSkewT(3, -0.3) # convenience constructor
StdSkewT{Float64}(coefs=[3.0, -0.3])
julia> StdGED(1) # convenience constructor
StdGED{Float64}(coefs=[1.0])

### User-defined standardized distributions

Apart from the natively supported standardized distributions, it is possible to wrap a continuous univariate distribution from the Distributions package in the Standardized wrapper type. Below, we reimplement the standardized normal distribution:

julia> using Distributions

julia> const MyStdNormal = Standardized{Normal};

MyStdNormal can be used whereever a built-in distribution could, albeit with a speed penalty. Note also that if the underlying distribution (such as Normal in the example above) contains location and/or scale parameters, then these are no longer identifiable, which implies that the estimated covariance matrix of the estimators will be singular.

A final remark concerns the domain of the parameters: the estimation process relies on a starting value for the parameters of the distribution, say $\theta\equiv(\theta_1, \ldots, \theta_p)'$. For a distribution wrapped in Standardized, the starting value for $\theta_i$ is taken to be a small positive value ϵ. This will fail if ϵ is not in the domain of $\theta_i$; as an example, the standardized Student's $t$ distribution is only defined for degrees of freedom larger than 2, because a finite variance is required for standardization. In that case, it is necessary to define a method of the (non-exported) function startingvals that returns a feasible vector of starting values, as follows:

julia> const MyStdT = Standardized{TDist};

julia> ARCHModels.startingvals(::Type{<:MyStdT}, data::Vector{T}) where T = T[3.]

## Working with UnivariateARCHModels

The constructor for UnivariateARCHModel takes two mandatory arguments: an instance of a subtype of UnivariateVolatilitySpec, and a vector of returns. The mean specification and error distribution can be changed via the keyword arguments meanspec and dist, which respectively default to NoIntercept and StdNormal.

For example, to construct a GARCH(1, 1) model with an intercept and $t$-distributed errors, one would do

julia> spec = GARCH{1, 1}([1., .9, .05]);

julia> data = BG96;

julia> am = UnivariateARCHModel(spec, data; dist=StdT(3.), meanspec=Intercept(1.))

GARCH{1, 1} model with Student's t errors, T=1974.

──────────────────────────────
μ
──────────────────────────────
Mean equation parameters:  1.0
──────────────────────────────
─────────────────────────────────────────
ω   β₁    α₁
─────────────────────────────────────────
Volatility parameters:     1.0  0.9  0.05
─────────────────────────────────────────
──────────────────────────────
ν
──────────────────────────────
Distribution parameters:   3.0
──────────────────────────────

The model can then be fitted as follows:

julia> fit!(am)

GARCH{1, 1} model with Student's t errors, T=1974.

Mean equation parameters:
─────────────────────────────────────────────
Estimate   Std.Error   z value  Pr(>|z|)
─────────────────────────────────────────────
μ  0.00227251  0.00686802  0.330882    0.7407
─────────────────────────────────────────────

Volatility parameters:
──────────────────────────────────────────────
Estimate   Std.Error   z value  Pr(>|z|)
──────────────────────────────────────────────
ω   0.00232225  0.00163909   1.41679    0.1565
β₁  0.884488    0.036963    23.929      <1e-99
α₁  0.124866    0.0405471    3.07952    0.0021
──────────────────────────────────────────────

Distribution parameters:
─────────────────────────────────────────
Estimate  Std.Error  z value  Pr(>|z|)
─────────────────────────────────────────
ν   4.11211   0.400384  10.2704    <1e-24
─────────────────────────────────────────

It should, however, rarely be necessary to construct a UnivariateARCHModel manually via its constructor; typically, instances of it are created by calling fit, selectmodel, or simulate.

Note

If you do manually construct a UnivariateARCHModel, be aware that the constructor does not create copies of its arguments. This means that, e.g., calling simulate! on the constructed model will modify your data vector:

julia> mydata = copy(BG96); mydata[end]
0.528047

julia> am = UnivariateARCHModel(ARCH{0}([1.]), mydata);

julia> simulate!(am);

julia> mydata[end] ≈ 0.528047
false

As discussed earlier, UnivariateARCHModel implements the interface of StatisticalModel from StatsBase, so you can call coef, coefnames, confint, dof, informationmatrix, isfitted, loglikelihood, nobs, score, stderror, vcov, etc. on its instances:

julia> nobs(am)
1974

Other useful methods include means, volatilities and residuals.