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Chapter Contents
Chapter Introduction
NAG Toolbox

NAG Toolbox: nag_tsa_uni_garch_asym1_estim (g13fa)

 Contents

    1  Purpose
    2  Syntax
    7  Accuracy
    9  Example

Purpose

nag_tsa_uni_garch_asym1_estim (g13fa) estimates the parameters of either a standard univariate regression GARCH process, or a univariate regression-type I AGARCHp,q process (see Engle and Ng (1993)).

Syntax

[theta, se, sc, covr, hp, et, ht, lgf, ifail] = g13fa(dist, yt, x, ip, iq, nreg, mn, isym, theta, hp, copts, maxit, tol, 'num', num, 'npar', npar)
[theta, se, sc, covr, hp, et, ht, lgf, ifail] = nag_tsa_uni_garch_asym1_estim(dist, yt, x, ip, iq, nreg, mn, isym, theta, hp, copts, maxit, tol, 'num', num, 'npar', npar)
Note: the interface to this routine has changed since earlier releases of the toolbox:
At Mark 25: nreg was made optional

Description

A univariate regression-type I AGARCHp,q process, with q coefficients αi, for i=1,2,,q, p coefficients βi, for i=1,2,,p, and k linear regression coefficients bi, for i=1,2,,k, can be represented by:
yt = bo + xtT b + εt (1)
ht=α0+i=1qαi εt-i+γ 2+i=1pβiht-i,  t=1,2,,T (2)
where εtψt-1=N0,ht or εtψt-1=Stdf,ht. Here St is a standardized Student's t-distribution with df degrees of freedom and variance ht, T is the number of terms in the sequence, yt denotes the endogenous variables, xt the exogenous variables, bo the regression mean, b the regression coefficients, εt the residuals, ht the conditional variance, df the number of degrees of freedom of the Student's t-distribution, and ψt the set of all information up to time t.
nag_tsa_uni_garch_asym1_estim (g13fa) provides an estimate for θ^, the parameter vector θ=bo,bT,ωT where bT=b1,,bk, ωT=α0,α1,,αq,β1,,βp,γ when dist='N' and ωT=α0,α1,,αq,β1,,βp,γ,df when dist='T'.
isym, mn and nreg can be used to simplify the GARCHp,q expression in (1) as follows:
No Regression and No Mean
No Regression
Note:  if the yt=μ+εt, where μ is known (not to be estimated by nag_tsa_uni_garch_asym1_estim (g13fa)) then (1) can be written as ytμ=εt, where ytμ=yt-μ. This corresponds to the case No Regression and No Mean, with yt replaced by yt-μ.
No Mean

References

Bollerslev T (1986) Generalised autoregressive conditional heteroskedasticity Journal of Econometrics 31 307–327
Engle R (1982) Autoregressive conditional heteroskedasticity with estimates of the variance of United Kingdom inflation Econometrica 50 987–1008
Engle R and Ng V (1993) Measuring and testing the impact of news on volatility Journal of Finance 48 1749–1777
Hamilton J (1994) Time Series Analysis Princeton University Press

Parameters

Compulsory Input Parameters

1:     dist – string (length ≥ 1)
The type of distribution to use for et.
dist='N'
A Normal distribution is used.
dist='T'
A Student's t-distribution is used.
Constraint: dist='N' or 'T'.
2:     ytnum – double array
The sequence of observations, yt, for t=1,2,,T.
3:     xldx: – double array
The first dimension of the array x must be at least num.
The second dimension of the array x must be at least nreg.
Row t of x must contain the time dependent exogenous vector xt , where xtT = xt1,,xtk , for t=1,2,,T.
4:     ip int64int32nag_int scalar
The number of coefficients, βi, for i=1,2,,p.
Constraint: ip0 (see also npar).
5:     iq int64int32nag_int scalar
The number of coefficients, αi, for i=1,2,,q.
Constraint: iq1 (see also npar).
6:     nreg int64int32nag_int scalar
k, the number of regression coefficients.
Constraint: nreg0 (see also npar).
7:     mn int64int32nag_int scalar
If mn=1, the mean term b0 will be included in the model.
Constraint: mn=0 or 1.
8:     isym int64int32nag_int scalar
If isym=1, the asymmetry term γ will be included in the model.
Constraint: isym=0 or 1.
9:     thetanpar – double array
The initial parameter estimates for the vector θ.
The first element must contain the coefficient αo and the next iq elements must contain the coefficients αi, for i=1,2,,q.
The next ip elements must contain the coefficients βj, for j=1,2,,p.
If isym=1, the next element must contain the asymmetry parameter γ.
If dist='T', the next element must contain df, the number of degrees of freedom of the Student's t-distribution.
If mn=1, the next element must contain the mean term bo.
If copts2=false, the remaining nreg elements are taken as initial estimates of the linear regression coefficients bi, for i=1,2,,k.
10:   hp – double scalar
If copts2=false, hp is the value to be used for the pre-observed conditional variance; otherwise hp is not referenced.
11:   copts2 – logical array
The options to be used by nag_tsa_uni_garch_asym1_estim (g13fa).
copts1=true
Stationary conditions are enforced, otherwise they are not.
copts2=true
The function provides initial parameter estimates of the regression terms, otherwise these are to be provided by you.
12:   maxit int64int32nag_int scalar
The maximum number of iterations to be used by the optimization function when estimating the GARCHp,q parameters. If maxit is set to 0, the standard errors, score vector and variance-covariance are calculated for the input value of θ in theta when dist='N'; however the value of θ is not updated.
Constraint: maxit0.
13:   tol – double scalar
The tolerance to be used by the optimization function when estimating the GARCHp,q parameters.

Optional Input Parameters

1:     num int64int32nag_int scalar
Default: the dimension of the array yt and the first dimension of the array x. (An error is raised if these dimensions are not equal.)
T, the number of terms in the sequence.
Constraints:
  • nummaxip,iq;
  • numnreg+mn.
2:     npar int64int32nag_int scalar
Default: the dimension of the array theta.
The number of parameters to be included in the model. npar=1+iq+ip+isym+mn+nreg when dist='N', and npar=2+iq+ip+isym+mn+nreg when dist='T'.
Constraint: npar<20.

Output Parameters

1:     thetanpar – double array
The estimated values θ^ for the vector θ.
The first element contains the coefficient αo, the next iq elements contain the coefficients αi, for i=1,2,,q.
The next ip elements are the coefficients βj, for j=1,2,,p.
If isym=1, the next element contains the estimate for the asymmetry parameter γ.
If dist='T', the next element contains an estimate for df, the number of degrees of freedom of the Student's t-distribution.
If mn=1, the next element contains an estimate for the mean term bo.
The final nreg elements are the estimated linear regression coefficients bi, for i=1,2,,k.
2:     senpar – double array
The standard errors for θ^.
The first element contains the standard error for αo. The next iq elements contain the standard errors for αi, for i=1,2,,q. The next ip elements are the standard errors for βj, for j=1,2,,p.
If isym=1, the next element contains the standard error for γ.
If dist='T', the next element contains the standard error for df, the number of degrees of freedom of the Student's t-distribution.
If mn=1, the next element contains the standard error for bo.
The final nreg elements are the standard errors for bj, for j=1,2,,k.
3:     scnpar – double array
The scores for θ^.
The first element contains the score for αo.
The next iq elements contain the score for αi, for i=1,2,,q.
The next ip elements are the scores for βj, for j=1,2,,p.
If isym=1, the next element contains the score for γ.
If dist='T', the next element contains the score for df, the number of degrees of freedom of the Student's t-distribution.
If mn=1, the next element contains the score for bo.
The final nreg elements are the scores for bj, for j=1,2,,k.
4:     covrldcovrnpar – double array
The covariance matrix of the parameter estimates θ^, that is the inverse of the Fisher Information Matrix.
5:     hp – double scalar
If copts2=true, hp is the estimated value of the pre-observed conditional variance.
6:     etnum – double array
The estimated residuals, εt, for t=1,2,,T.
7:     htnum – double array
The estimated conditional variances, ht, for t=1,2,,T.
8:     lgf – double scalar
The value of the log-likelihood function at θ^.
9:     ifail int64int32nag_int scalar
ifail=0 unless the function detects an error (see Error Indicators and Warnings).

Error Indicators and Warnings

Note: nag_tsa_uni_garch_asym1_estim (g13fa) may return useful information for one or more of the following detected errors or warnings.
Errors or warnings detected by the function:

Cases prefixed with W are classified as warnings and do not generate an error of type NAG:error_n. See nag_issue_warnings.

   ifail=1
On entry,nreg<0,
ormn>1,
ormn<0,
orisym>1,
orisym<0,
oriq<1,
orip<0,
ornpar20,
ornpar has an invalid value,
orldcovr<npar,
orldx<num,
ordist'N',
ordist'T',
ormaxit<0,
ornum<maxip,iq,
ornum<nreg+mn.
   ifail=2
On entry,lwork<nreg+3×num+npar+403.
   ifail=3
The matrix X is not full rank.
   ifail=4
The information matrix is not positive definite.
   ifail=5
The maximum number of iterations has been reached.
W  ifail=6
The log-likelihood cannot be optimized any further.
   ifail=7
No feasible model parameters could be found.
   ifail=-99
An unexpected error has been triggered by this routine. Please contact NAG.
   ifail=-399
Your licence key may have expired or may not have been installed correctly.
   ifail=-999
Dynamic memory allocation failed.

Accuracy

Not applicable.

Further Comments

None.

Example

This example fits a GARCH1,1 model with Student's t-distributed residuals to some simulated data.
The process parameter estimates, θ^, are obtained using nag_tsa_uni_garch_asym1_estim (g13fa), and a four step ahead volatility estimate is computed using nag_tsa_uni_garch_asym1_forecast (g13fb).
The data was simulated using nag_rand_times_garch_asym1 (g05pd).
function g13fa_example


fprintf('g13fa example results\n\n');

num  = 100;

% Series
yt = [ 9.04;  9.49;  9.12;  9.23;  9.35;
       9.09;  9.75;  9.23;  8.76;  9.17;
       9.20;  9.64;  8.74;  9.23;  9.42;
       9.70;  9.55; 10.00;  9.18;  9.77;
       9.80;  9.56;  9.28;  9.68;  9.51;
       9.51;  8.97;  9.30;  9.52;  9.41;
       9.53;  9.75;  9.72;  9.38;  9.28;
       9.42;  9.74;  9.75;  9.60;  9.90;
       9.06;  9.92;  9.21;  9.57;  9.42;
       8.65;  8.85;  9.61; 10.77; 10.19;
      10.47; 10.10; 10.21;  9.96;  9.66;
       9.79; 10.30;  9.68; 10.08; 10.38;
       9.69;  9.02;  9.89; 10.46; 10.47;
       9.99;  9.76;  9.78;  9.62; 10.43;
      10.42;  9.95;  9.95;  9.70; 10.24;
       9.78;  9.98;  8.73; 10.23;  9.10;
      10.27;  9.85; 10.44; 10.30; 10.08;
      10.20; 10.14;  9.89;  9.90; 11.33;
       9.71;  9.40;  9.97; 10.92;  9.76;
      10.16; 10.43;  9.60; 10.29; 10.03];

% Exogenous variables
x = [0.12, 2.40; 0.12, 2.40; 0.13, 2.40; 0.14, 2.40; 0.14, 2.40;
     0.15, 2.40; 0.16, 2.40; 0.16, 2.40; 0.17, 2.40; 0.18, 2.41;
     0.19, 2.41; 0.19, 2.41; 0.20, 2.41; 0.21, 2.41; 0.21, 2.41;
     0.22, 2.41; 0.23, 2.41; 0.23, 2.41; 0.24, 2.41; 0.25, 2.42;
     0.25, 2.42; 0.26, 2.42; 0.26, 2.42; 0.27, 2.42; 0.28, 2.42;
     0.28, 2.42; 0.29, 2.42; 0.30, 2.42; 0.30, 2.42; 0.31, 2.43;
     0.32, 2.43; 0.32, 2.43; 0.33, 2.43; 0.33, 2.43; 0.34, 2.43;
     0.35, 2.43; 0.35, 2.43; 0.36, 2.43; 0.37, 2.43; 0.37, 2.44;
     0.38, 2.44; 0.38, 2.44; 0.39, 2.44; 0.39, 2.44; 0.40, 2.44;
     0.41, 2.44; 0.41, 2.44; 0.42, 2.44; 0.42, 2.44; 0.43, 2.45;
     0.43, 2.45; 0.44, 2.45; 0.45, 2.45; 0.45, 2.45; 0.46, 2.45;
     0.46, 2.45; 0.47, 2.45; 0.47, 2.45; 0.48, 2.45; 0.48, 2.46;
     0.49, 2.46; 0.49, 2.46; 0.50, 2.46; 0.50, 2.46; 0.51, 2.46;
     0.51, 2.46; 0.52, 2.46; 0.52, 2.46; 0.53, 2.46; 0.53, 2.47;
     0.54, 2.47; 0.54, 2.47; 0.54, 2.47; 0.55, 2.47; 0.55, 2.47;
     0.56, 2.47; 0.56, 2.47; 0.57, 2.47; 0.57, 2.47; 0.57, 2.48;
     0.58, 2.48; 0.58, 2.48; 0.59, 2.48; 0.59, 2.48; 0.59, 2.48;
     0.60, 2.48; 0.60, 2.48; 0.61, 2.48; 0.61, 2.48; 0.61, 2.49;
     0.62, 2.49; 0.62, 2.49; 0.62, 2.49; 0.63, 2.49; 0.63, 2.49;
     0.63, 2.49; 0.64, 2.49; 0.64, 2.49; 0.64, 2.49; 0.64, 2.50];

% Details of model to fit
dist = 't';
n1 = int64(1);
ip = n1;
iq = n1;
isym = n1;
mn   = n1;
nreg = 2*n1;

% Control parameters
copts = [true; true];
maxit = int64(200);
tol = 0.00001;

% Initial values
gammaval = -0.1;
theta = [0.05; 0.1; 0.15; gammaval; 2.6; 1.5; 0; 0];

% Forecast horizon
nt = 4*n1;

% Fit the GARCH model
[theta, se, sc, covar, hp, et, ht, lgf, ifail] = ...
  g13fa( ...
         dist, yt, x, ip, iq, mn, isym, theta, 0, copts, maxit, tol);

% Calculate the volatility forecast
[fht, ifail] = g13fb( ...
                      nt, ip, iq, theta, gammaval, ht, et);

% Output the results
fprintf('\n               Parameter        Standard\n');
fprintf('               estimates         errors\n');

% Output the coefficient alpha_0
fprintf('Alpha0 %16.2f%16.2f\n', theta(1), se(1));
l = 2;

% Output the coefficients alpha_i
for i = l:l+iq-1
  fprintf('Alpha%d %16.2f%16.2f\n', i-1, theta(i), se(i));
end
l = l+iq;

% Output the coefficients beta_j
fprintf('\n');
for i = l:l+ip-1
  fprintf(' Beta%d %16.2f%16.2f\n', i-l+1, theta(i), se(i));
end
l = l+ip;

% Output the estimated asymmetry parameter, gamma
if (isym == 1)
  fprintf('\n Gamma %16.2f%16.2f\n', theta(l), se(l));
  l = l+1;
end
% Output the estimated degrees of freedom, df
if (dist == 't')
  fprintf('\n    DF %16.2f%16.2f\n', theta(l), se(l));
  l = l + 1;
end

% Output the estimated mean term, b_0
if (mn == 1)
  fprintf('\n    B0 %16.2f%16.2f\n', theta(l), se(l));
  l = l + 1;
end

% Output the estimated linear regression coefficients, b_i
for i = l:l+nreg-1
  fprintf('    B%d %16.2f%16.2f\n', i-l+1, theta(i), se(i));
end

% Display the volatility forecast
fprintf('\nVolatility forecast = %12.4f\n', fht(nt));


g13fa example results


               Parameter        Standard
               estimates         errors
Alpha0             0.00            0.06
Alpha1             0.11            0.13

 Beta1             0.66            0.23

 Gamma            -0.62            0.62

    DF             6.25            4.70

    B0             3.85           24.11
    B1             1.48            1.82
    B2             2.15           10.16

Volatility forecast =       0.0626

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