forecast

Forecast conditional variances from conditional variance models

Syntax

V = forecast(Mdl,numperiods,Y0)

Tbl2 = forecast(Mdl,numperiods,Tbl1)

[___] = forecast(___,Name,Value)

Description

V = forecast(Mdl,numperiods,Y0) returns a numeric array containing paths of minimum mean squared error (MMSE), consecutive forecasted conditional variances V of the fully specified, univariate conditional variance model Mdl, over a numperiods forecast horizon. The model Mdl can be a garch, egarch, or gjr model object. The forecasts represent the continuation of the conditional variances associated with the presample data in the numeric array Y0.

Tbl2 = forecast(Mdl,numperiods,Tbl1) returns the table or timetable Tbl2 containing the paths of MMSE conditional variance variable forecasts of the model Mdl over a numperiods forecast horizon. forecast uses the table or timetable of presample data Tbl1 to initialize the response series. (since R2023a)

To initialize the forecast, forecast selects the response variable named in Mdl.SeriesName or the sole variable in Tbl1. To select a different response variable in Tbl1 to initialize the forecasts, use the PresampleResponseVariable name-value argument.

[___] = forecast(___,Name,Value) specifies options using one or more name-value arguments in addition to any of the input argument combinations in previous syntaxes. forecast returns the output argument combination for the corresponding input arguments. For example, forecast(Mdl,10,Y0,V0=v0) initializes the conditional variances for the forecast using the presample data in v0.

Examples

Specify Numeric Presample Response Data to Forecast GARCH Model Conditional Variances

Forecast the conditional variance of simulated data over a 30-period horizon. Supply a vector of presample response data.

Simulate 100 observations from a GARCH(1,1) model with known parameters.

Mdl = garch(Constant=0.02,GARCH=0.8,ARCH=0.1);
rng("default") % For reproducibility
[v,y] = simulate(Mdl,100);

Forecast the conditional variances over a 30-period horizon. Specify the simulated response data. Plot the forecasts.

vF = forecast(Mdl,30,y);

figure
plot(v,"-b")
hold on
plot(101:130,vF,"r--",LineWidth=2);
title("Forecasted Conditional Variances")
legend("Simulated presample","Forecasts")
hold off

Forecasts converge asymptotically to the unconditional innovation variance.

Forecast EGARCH Model Conditional Variances

Forecast the conditional variance of simulated data over a 30-period horizon.

Simulate 100 observations from an EGARCH(1,1) model with known parameters.

Mdl = egarch(Constant=0.01,GARCH=0.6,ARCH=0.2, ...
    Leverage=-0.2);
rng("default") % For reproducibility
[v,y] = simulate(Mdl,100);

Forecast the conditional variance over a 30-period horizon. Specify the simulated data as presample responses. Plot the forecasts.

VF1 = forecast(Mdl,30,y);

figure
plot(v,"r-")
hold on
plot(101:130,VF1,"b--",LineWidth=2);
title("Forecasted Conditional Variances")
legend("Simulated responses","Forecasts")
hold off

Forecast GJR Model Conditional Variances

Forecast the conditional variance of simulated data over a 30-period horizon.

Simulate 100 observations from a GJR(1,1) model with known parameters.

Mdl = gjr(Constant=0.01,GARCH=0.6,ARCH=0.2, ...
    Leverage=0.2);
rng("default") % For reproducibility
[v,y] = simulate(Mdl,100);

Forecast the conditional variances over a 30-period horizon. Specify the simulated presample responses. Plot the forecasts.

vF = forecast(Mdl,30,y);

figure
plot(v,"r")
hold on
plot(101:130,vF,'b--',LineWidth=2);
title("Forecasted Conditional Variances")
legend("Observed","Forecasts")
hold off

Compare Conditional Variance Forecasts of NYSE Returns

Since R2023a

Forecast the conditional variance of the average weekly closing NASDAQ returns from fitted GARCH(1,1), EGARCH(1,1) and GJR(1,1) models.

Load the U.S. equity indices data Data_EquityIdx.mat.

load Data_EquityIdx

The timetable DataTimeTable contains the daily NASDAQ closing prices, among other indices.

Compute the weekly average closing prices of all timetable variables.

DTTW = convert2weekly(DataTimeTable,Aggregation="mean");

Compute the weekly percent returns and their sample mean.

DTTRet = price2ret(DTTW);
DTTRet.Interval = [];
DTTRet.NASDAQ = DTTRet.NASDAQ*100;
T = height(DTTRet)

T = 
626

meanRet = mean(DTTRet.NASDAQ)

meanRet = 
0.0330

figure
plot(DTTRet.Time,100*DTTRet.NASDAQ);
hold on
yline(100*meanRet,'--r')
title("Daily NASDAQ Returns");
xlabel("Date");
ylabel("Return (%)");

The variance of the series seems to change. This change is an indication of volatility clustering. The conditional mean model offset is very close to zero.

When you plan to supply a timetable, you must ensure it has all the following characteristics:

• The selected response variable is numeric and does not contain any missing values.

• The timestamps in the Time variable are regular, and they are ascending or descending.

Remove all missing values from the timetable, relative to the NASDAQ returns series.

DTTRet = rmmissing(DTTRet,DataVariables="NASDAQ");
numobs = height(DTTRet)

numobs = 
626

Because all sample times have observed NASDAQ returns, rmmissing does not remove any observations.

Determine whether the sampling timestamps have a regular frequency and are sorted.

areTimestampsRegular = isregular(DTTRet,"weeks")

areTimestampsRegular = logical
   1

areTimestampsSorted = issorted(DTTRet.Time)

areTimestampsSorted = logical
   1

areTimestampsRegular = 1 indicates that the timestamps of DTTRet represent a regular weekly sample. areTimestampsSorted = 1 indicates that the timestamps are sorted.

Fit GARCH(1,1), EGARCH(1,1), and GJR(1,1) models to the data. By default, the software sets the conditional mean model offset to zero.

MdlGARCH = garch(1,1);
MdlEGARCH = egarch(1,1);
MdlGJR = gjr(1,1);

EstMdlGARCH = estimate(MdlGARCH,DTTRet,ResponseVariable="NASDAQ");  

 
    GARCH(1,1) Conditional Variance Model (Gaussian Distribution):
 
                  Value      StandardError    TStatistic      PValue   
                _________    _____________    __________    ___________

    Constant    0.0030629      0.0011827        2.5897        0.0096065
    GARCH{1}      0.86501        0.02911        29.715      4.8912e-194
    ARCH{1}       0.11835       0.024582        4.8144       1.4765e-06

EstMdlEGARCH = estimate(MdlEGARCH,DTTRet,ResponseVariable="NASDAQ");  

 
    EGARCH(1,1) Conditional Variance Model (Gaussian Distribution):
 
                     Value      StandardError    TStatistic      PValue  
                   _________    _____________    __________    __________

    Constant       -0.081262      0.030237        -2.6875       0.0071983
    GARCH{1}         0.95557       0.01335         71.579               0
    ARCH{1}           0.2768      0.052237          5.299      1.1645e-07
    Leverage{1}     -0.10519      0.025542        -4.1185      3.8142e-05

EstMdlGJR = estimate(MdlGJR,DTTRet,ResponseVariable="NASDAQ");

 
    GJR(1,1) Conditional Variance Model (Gaussian Distribution):
 
                     Value      StandardError    TStatistic      PValue  
                   _________    _____________    __________    __________

    Constant       0.0069063      0.0020036         3.447       0.0005668
    GARCH{1}         0.78545       0.043862        17.907      1.0334e-71
    ARCH{1}         0.090637       0.034313        2.6415       0.0082543
    Leverage{1}      0.18663       0.054402        3.4305       0.0006025

Forecast the conditional variance for 20 weeks using the fitted models. Use the observed returns as presample innovations for the forecasts.

fh = 20;
DTTVFGARCH = forecast(EstMdlGARCH,fh,DTTRet, ...
    PresampleResponseVariable="NASDAQ");  
DTTVFEGARCH = forecast(EstMdlEGARCH,fh,DTTRet, ...
    PresampleResponseVariable="NASDAQ");  
DTTVFGJR= forecast(EstMdlGJR,fh,DTTRet, ...
    PresampleResponseVariable="NASDAQ");

The forecasted conditional variance variables are called Y_Variance in each returned timetable.

Plot the forecasts along with the conditional variances inferred from the data.

DTTVGARCH = infer(EstMdlGARCH,DTTRet,ResponseVariable="NASDAQ");
DTTVEGARCH = infer(EstMdlEGARCH,DTTRet,ResponseVariable="NASDAQ");
DTTVGJR = infer(EstMdlGJR,DTTRet,ResponseVariable="NASDAQ");

figure
tiledlayout(3,1)
nexttile
plot(DTTRet.Time(end-100:end),DTTVGARCH.Y_Variance(end-100:end), ...
    "r",DTTVFGARCH.Time,DTTVFGARCH.Y_Variance,"b--")
legend("Inferred","Forecast",Location="northeast")
title("GARCH(1,1) Conditional Variances")
nexttile
plot(DTTRet.Time(end-100:end),DTTVEGARCH.Y_Variance(end-100:end),"r", ...
    DTTVFEGARCH.Time,DTTVFEGARCH.Y_Variance,"b--")
legend("Inferred","Forecast",Location="northeast")
title("EGARCH(1,1) Conditional Variances")
nexttile
plot(DTTRet.Time(end-100:end),DTTVGJR.Y_Variance(end-100:end),"r", ...
    DTTVFGJR.Time,DTTVFGJR.Y_Variance,'b--')
legend("Inferred","Forecast",Location="northeast")
title("GJR(1,1) Conditional Variances")

Plot conditional variance forecasts for the next 500 weeks after the sample.

fh = 500;
DTTVF1000GARCH = forecast(EstMdlGARCH,fh,DTTRet, ...
    PresampleResponseVariable="NASDAQ");  
DTTVF1000EGARCH = forecast(EstMdlEGARCH,fh,DTTRet, ...
    PresampleResponseVariable="NASDAQ");  
DTTVF1000GJR= forecast(EstMdlGJR,fh,DTTRet, ...
    PresampleResponseVariable="NASDAQ");
figure
plot(DTTVF1000GARCH.Time,DTTVF1000GARCH.Y_Variance,'b',...
    DTTVF1000EGARCH.Time,DTTVF1000EGARCH.Y_Variance,'r',...
    DTTVF1000GJR.Time,DTTVF1000GJR.Y_Variance,'k')
legend("GARCH Forecast","EGARCH Foecast","GJR Forecast",Location="northeast")
title("Long-Run Conditional Variance Forecast")

The forecasts converge asymptotically to the unconditional variances of their respective processes.

Input Arguments

Mdl — Conditional variance model

garch model object | egarch model object | gjr model object

Conditional variance model without any unknown parameters, specified as a garch, egarch, or gjr model object.

Mdl cannot contain any properties that have NaN value.

numperiods — Forecast horizon

positive integer

Forecast horizon, or the number of time points in the forecast period, specified as a positive integer.

Data Types: double

Y0 — Presample response data y_t

numeric column vector | numeric matrix

Presample response data y_t used to infer presample innovations ε_t, and whose conditional variance process σ_t² is forecasted, specified as a numpreobs-by-1 numeric column vector or a numpreobs-by-numpaths numeric matrix. When you supply Y0, supply all optional data as numeric arrays, and forecast returns results in numeric arrays.

numpreobs is the number of presample observations.

Y0 can represent a mean 0 presample innovations series with a variance process characterized by the conditional variance model Mdl. Y0 can also represent a presample innovations series plus an offset (stored in Mdl.Offset). For more details, see Algorithms.

Each row is a presample observation, and measurements in each row occur simultaneously. The last row contains the latest presample observation. numpreobs must be at least Mdl.Q to initialize the conditional variance model. If numpreobs > Mdl.Q, forecast uses only the latest Mdl.Q rows. For more details, see Time Base Partitions for Forecasting.

Columns of Y0 correspond to separate, independent paths.

• If Y0 is a column vector, it represents a single path of the response series. forecast applies it to each forecasted path. In this case, all forecast paths Y derive from the same initial responses.

• If Y0 is a matrix, each column represents a presample path of the response series. numpaths is the maximum among the second dimensions of the specified presample observation matrices Y0 and V0.

Data Types: double

Tbl1 — Presample data

table | timetable

Since R2023a

Presample data containing the response variable y_t and, optionally, the conditional variance variable σ_t² used to initialize the model for the forecast, specified as a table or timetable, the same type as Tbl1, with numprevars variables and numpreobs rows. You can select a response variable or conditional variance variable from Tbl1 by using the PresampleResponseVariable or PresampleVarianceVariable name-value argument, respectively.

Each selected variable is single path (numpreobs-by-1 vector) or multiple paths (numpreobs-by-numpaths matrix) of presample response or conditional variance data. Each row is a presample observation, and measurements in each row occur simultaneously. numpreobs must be one of the following values:

• Mdl.Q when Tbl1 provides only presample responses

• max([Mdl.P Mdl.Q]) when Tbl1 also provides presample conditional variances

If you supply more rows than necessary, forecast uses the latest required number of observations only.

If Tbl1 is a timetable, all the following conditions must be true:

• Tbl1 must represent a sample with a regular datetime time step (see isregular).

• The datetime vector of sample timestamps Tbl1.Time must be ascending or descending.

If Tbl1 is a table, the last row contains the latest presample observation.

Although forecast requires presample response data, forecast sets default presample conditional variance data in one of the following ways:

• If numpreobs ≥ max([Mdl.P Mdl.Q]) + Mdl.P, forecast infers presample conditional variances from the presample response data (see infer).

• Otherwise:

  • If Mdl is a GARCH(P,Q) or GJR(P,Q) model, forecast sets all required conditional variances to the unconditional variance of the conditional variance process.

  • If Mdl is a EGARCH(P,Q) model, forecast sets all required conditional variances to the exponentiated, unconditional mean of the logarithm of the EGARCH(P,Q) variance process.

Name-Value Arguments

Specify optional pairs of arguments as Name1=Value1,...,NameN=ValueN, where Name is the argument name and Value is the corresponding value. Name-value arguments must appear after other arguments, but the order of the pairs does not matter.

Before R2021a, use commas to separate each name and value, and enclose Name in quotes.

Example: forecast(Mdl,10,Y0,V0=[1 0.5;1 0.5]) specifies two different presample paths of conditional variances.

V0 — Presample conditional variances σ_t²

positive column vector | positive matrix

Presample conditional variances σ_t² used to initialize the conditional variance model, specified as a numpreobs-by-1 positive column vector or numpreobs-by-numpaths positive matrix. Use V0 only when you supply the numeric array of presample response data Y0.

Rows of V0 correspond to periods in the presample, and the last row contains the latest presample conditional variance.

• For GARCH(P,Q) and GJR(P,Q) models, numpreobs must be at least Mdl.P to initialize the variance equation.

• For EGARCH(P,Q) models, numpreobs must be at least max([Mdl.P Mdl.Q]) to initialize the variance equation.

If numpreobs exceeds the minimum number, forecast uses only the latest observations.

Columns of V0 correspond to separate, independent paths.

• If V0 is a column vector, forecast applies it to each forecasted path. In this case, the conditional variance model of all forecast paths V derives from the same initial conditional variances.

• If V0 is a matrix, it must have numpaths columns, the same number of columns as Y0.

forecast sets default presample conditional variance data in one of the following ways:

• If the number of rows of Y0 numpreobs ≥ max([Mdl.P Mdl.Q]) + Mdl.P, forecast infers V0 from Y0 (see infer).

• Otherwise:

  • If Mdl is a GARCH(P,Q) or GJR(P,Q) model, forecast sets all required conditional variances to the unconditional variance of the conditional variance process.

  • If Mdl is a EGARCH(P,Q) model, forecast sets all required conditional variances to the exponentiated, unconditional mean of the logarithm of the EGARCH(P,Q) variance process.

Data Types: double

PresampleResponseVariable — Variable of Tbl1 containing presample response paths y_t

string scalar | character vector | integer | logical vector

Since R2023a

Variable of Tbl1 containing presample response paths y_t, specified as one of the following data types:

• String scalar or character vector containing a variable name in Tbl1.Properties.VariableNames

• Variable index (integer) to select from Tbl1.Properties.VariableNames

• A length numprevars logical vector, where PresampleResponseVariable(j) = true selects variable j from Tbl1.Properties.VariableNames, and sum(PresampleResponseVariable) is 1

The selected variable must be a numeric matrix and cannot contain missing values (NaN).

If Tbl1 has one variable, the default specifies that variable. Otherwise, the default matches the variable to name in Mdl.SeriesName.

Example: PresampleResponseVariable="StockRate"

Example: PresampleResponseVariable=[false false true false] or PresampleResponseVariable=3 selects the third table variable as the presample response variable.

Data Types: double | logical | char | cell | string

PresampleVarianceVariable — Variable of Tbl1 containing presample conditional variance paths σ_t²

string scalar | character vector | integer | logical vector

Since R2023a

Variable of Tbl1 containing presample conditional variance paths σ_t², specified as one of the following data types:

• String scalar or character vector containing a variable name in Tbl1.Properties.VariableNames

• Variable index (integer) to select from Tbl1.Properties.VariableNames

• A length numprevars logical vector, where PresampleVarianceVariable(j) = true selects variable j from Tbl1.Properties.VariableNames, and sum(PresampleVarianceVariable) is 1

The selected variable must be a numeric vector and cannot contain missing values (NaN).

To use presample conditional variance data in Tbl1, you must specify PresampleVarianceVariable.

Example: PresampleVarianceVariable="StockRateVar"

Example: PresampleVarianceVariable=[false false true false] or PresampleVarianceVariable=3 selects the third table variable as the presample conditional variance variable.

Data Types: double | logical | char | cell | string

Note

• NaN values in numeric presample data sets Y0 and V0 indicate missing data. forecast removes missing data from the presample data sets following this procedure:

  1. forecast horizontally concatenates Y0 and V0 such that the latest observations occur simultaneously. The result can be a jagged array because the presample data sets can have a different number of rows. In this case, forecast prepads variables with an appropriate amount of zeros to form a matrix.

  2. forecast applies list-wise deletion to the combined presample matrix by removing all rows containing at least one NaN.

  3. forecast extracts the processed presample data sets from the result of step 2, and removes all prepadded zeros.

  List-wise deletion reduces the sample size and can create irregular time series.

• For numeric data inputs, forecast assumes that you synchronize the presample data such that the latest observations occur simultaneously.

• forecast issues an error when any table or timetable input contains missing values.

Output Arguments

V — Paths of MMSE forecasts of conditional variances σ_t² of future model innovations ε_t

numeric column vector | numeric matrix

Paths of MMSE forecasts of conditional variances σ_t² of future model innovations ε_t, returned as a numperiods-by-1 numeric column vector or a numperiods-by-numpaths numeric matrix. forecast returns V only when you supply the input Y0.

V represents a continuation of V0 (V(1,:) occurs in the next time point after V0(end,:)).

V(j,k) contains the j-period-ahead forecasted conditional variance of path k.

forecast determines numpaths from the number of columns in the presample data sets Y0 and V0. For details, see Algorithms. If each presample data set has one column, then V is a column vector.

Tbl2 — Paths of MMSE forecasts of conditional variances σ_t² of future model innovations ε_t

table | timetable

Since R2023a

Paths of MMSE forecasts of conditional variances σ_t² of future model innovations ε_t, returned as a table or timetable, the same data type as Tbl1. forecast returns Tbl2 only when you supply the input Tbl1.

Tbl2 contains a variable for all forecasted conditional variance paths, which are in a numperiods-by-numpaths numeric matrix, with rows representing periods in the forecast horizon and columns representing independent paths, each corresponding to the input presample response and conditional variance paths in Tbl1. forecast names the forecasted conditional variance variable in Tbl2 responseName_Variance, where responseName is Mdl.SeriesName. For example, if Mdl.SeriesName is StockReturns, Tbl2 contains a variable for the corresponding forecasted conditional variance paths with the name StockReturns_Variance.

Tbl2.responseName_Variance represents a continuation of the presample conditional variance process, either supplied by Tbl1 or set by default (Tbl2.responseName_Variance(1,:) occurs in the next time point, with respect to the periodicity Tbl1, after the last presample conditional variance).

Tbl2.responseName_Variance(j,k) contains the j-period-ahead forecasted conditional variance of path k.

If Tbl1 is a timetable, the following conditions hold:

• The row order of Tbl2, either ascending or descending, matches the row order of Tbl1.

• Tbl2.Time(1) is the next time after Tbl1.Time(end) relative the sampling frequency, and Tbl2.Time(2:numobs) are the following times relative to the sampling frequency.

More About

Time Base Partitions for Forecasting

Time base partitions for forecasting are two disjoint, contiguous intervals of the time base; each interval contains time series data for forecasting a dynamic model. The forecast period (forecast horizon) is a numperiods length partition at the end of the time base during which forecast generates forecasts V from the dynamic model Mdl. The presample period is the entire partition occurring before the forecast period. forecast can require observed responses (or innovations) Y0 or conditional variances V0 in the presample period to initialize the dynamic model for forecasting. The model structure determines the types and amounts of required presample observations.

A common practice is to fit a dynamic model to a portion of the data set, then validate the predictability of the model by comparing its forecasts to observed responses. During forecasting, the presample period contains the data to which the model is fit, and the forecast period contains the holdout sample for validation. Suppose that y_t is an observed response series. Consider forecasting conditional variances from a dynamic model of y_t numperiods = K periods. Suppose that the dynamic model is fit to the data in the interval [1,T – K] (for more details, see estimate). This figure shows the time base partitions for forecasting.

For example, to generate forecasts Y from a GARCH(0,2) model, forecast requires presample responses (innovations) Y0 = $${\left[\begin{array}{cc}{y}_{T-K-1}& y_{T-K}\end{array}\right]}^{\prime }$$ to initialize the model. The 1-period-ahead forecast requires both observations, whereas the 2-periods-ahead forecast requires y_{T – K} and the 1-period-ahead forecast V(1). forecast generates all other forecasts by substituting previous forecasts for lagged responses in the model.

Dynamic models containing a GARCH component can require presample conditional variances. Given enough presample responses, forecast infers the required presample conditional variances. This figure shows the arrays of required observations for this case, with corresponding input and output arguments.

Algorithms

• If the conditional variance model Mdl has an offset (Mdl.Offset), forecast subtracts it from the specified presample responses to obtain presample innovations. Subsequently, forecast uses to initialize the conditional variance model for forecasting.

• forecast sets the number of sample paths to forecast numpaths to the maximum number of columns among the specified presample response and conditional variance data sets. All presample data sets must have either numpaths > 1 columns or one column. Otherwise, forecast issues an error. For example, if Y0 has five columns, representing five paths, then V0 can either have five columns or one column. If V0 has one column, then forecast applies V0 to each path.

References

[1] Bollerslev, T. “Generalized Autoregressive Conditional Heteroskedasticity.” Journal of Econometrics. Vol. 31, 1986, pp. 307–327.

[2] Bollerslev, T. “A Conditionally Heteroskedastic Time Series Model for Speculative Prices and Rates of Return.” The Review of Economics and Statistics. Vol. 69, 1987, pp. 542–547.

[3] Box, G. E. P., G. M. Jenkins, and G. C. Reinsel. Time Series Analysis: Forecasting and Control. 3rd ed. Englewood Cliffs, NJ: Prentice Hall, 1994.

[4] Enders, W. Applied Econometric Time Series. Hoboken, NJ: John Wiley & Sons, 1995.

[5] Engle, R. F. “Autoregressive Conditional Heteroskedasticity with Estimates of the Variance of United Kingdom Inflation.” Econometrica. Vol. 50, 1982, pp. 987–1007.

[6] Glosten, L. R., R. Jagannathan, and D. E. Runkle. “On the Relation between the Expected Value and the Volatility of the Nominal Excess Return on Stocks.” The Journal of Finance. Vol. 48, No. 5, 1993, pp. 1779–1801.

[7] Hamilton, J. D. Time Series Analysis. Princeton, NJ: Princeton University Press, 1994.

[8] Nelson, D. B. “Conditional Heteroskedasticity in Asset Returns: A New Approach.” Econometrica. Vol. 59, 1991, pp. 347–370.

Version History

Introduced in R2012a

R2023a: forecast accepts input data in tables and timetables, and returns results in tables and timetables

In addition to accepting input presample data in numeric arrays, forecast accepts input data in tables or regular timetables. When you supply data in a table or timetable, the following conditions apply:

• forecast chooses the default series on which to operate, but you can use the specified optional name-value argument to select a different variable.

• forecast returns results in a table or timetable.

Name-value arguments to support tabular workflows include:

• PresampleResponseVariable specifies the variable name of the response paths in the input presample data Tbl1 to initialize the response series for the forecast.

• PresampleVarianceVariable specifies the variable name of the conditional variance paths in the input presample data Tbl1 to initialize the conditional variance series for the forecast.

R2019a: Models require specification of presample response data to forecast conditional variances

forecast now has a third input argument for you to supply presample response data.

forecast(Mdl,numperiods,Y0)
forecast(Mdl,numperiods,Y0,Name,Value)

Before R2019a, the syntaxes were:

forecast(Mdl,numperiods)
forecast(Mdl,numperiods,Name,Value)

You could optionally supply presample responses using the Y0 name-value argument.

There are no plans to remove the previous syntaxes or the Y0 name-value argument at this time. However, you are encouraged to supply presample responses because, to forecast conditional variances from a conditional variance model, forecast must initialize models containing lagged variables. Without specified presample responses, forecast initializes models by using reasonable default values, but the default might not support all workflows. This table describes the default values for each conditional variance model object.

Model Object	Presample Default
garch	All presample responses are the unconditional standard deviation of the process.
egarch	All presample responses are 0.
gjr	All presample responses are the unconditional standard deviation of the process.

Update Code

Update your code by specifying presample response data in the third input argument.

If you do not supply presample responses, forecast provides default presample values that might not support all workflows.