Asset Returns and Scenarios Using PortfolioMAD Object

How Stochastic Optimization Works

The MAD of a portfolio is mean-absolute deviation. For the definition of the MAD function, see Risk Proxy. Although analytic solutions for MAD exist for a few probability distributions, an alternative is to compute the expectation for MAD with samples from the probability distribution of asset returns. These samples are called scenarios and, given a collection of scenarios, the portfolio optimization problem becomes a stochastic optimization problem.

As a function of the portfolio weights, the MAD of the portfolio is a convex non-smooth function (see Konno and Yamazaki [50] at Portfolio Optimization). The PortfolioMAD object computes MAD as this nonlinear function which can be handled by the solver fmincon Optimization Toolbox™. The nonlinear programming solver fmincon has several algorithms that can be selected with the setSolver function, the two algorithms that work best in practice are 'sqp' and 'active-set'.

There are reformulations of the MAD portfolio optimization problem (see Konno and Yamazaki [50] at Portfolio Optimization) that result in a linear programming problem, which can be solved either with standard linear programming techniques or with stochastic programming solvers. The PortfolioMAD object, however, does not reformulate the problem in such a manner. The PortfolioMAD object computes the MAD as a nonlinear function. The convexity of the MAD, as a function of the portfolio weights and the dull edges when the number of scenarios is large, make the MAD portfolio optimization problem tractable, in practice, for certain nonlinear programming solvers, such as fmincon from Optimization Toolbox. To learn more about the workflow when using PortfolioMAD objects, see PortfolioMAD Object Workflow.

What Are Scenarios?

Since mean absolute deviation portfolio optimization works with scenarios of asset returns to perform the optimization, several ways exist to specify and simulate scenarios. In many applications with MAD portfolio optimization, asset returns may have distinctly nonnormal probability distributions with either multiple modes, binning of returns, truncation of distributions, and so forth. In other applications, asset returns are modeled as the result of various simulation methods that might include Monte-Carlo simulation, quasi-random simulation, and so forth. Often, the underlying probability distribution for risk factors may be multivariate normal but the resultant transformations are sufficiently nonlinear to result in distinctively nonnormal asset returns.

For example, this occurs with bonds and derivatives. In the case of bonds with a nonzero probability of default, such scenarios would likely include asset returns that are −100% to indicate default and some values slightly greater than −100% to indicate recovery rates.

Although the PortfolioMAD object has functions to simulate multivariate normal scenarios from either data or moments (simulateNormalScenariosByData and simulateNormalScenariosByMoments), the usual approach is to specify scenarios directly from your own simulation functions. These scenarios are entered directly as a matrix with a sample for all assets across each row of the matrix and with samples for an asset down each column of the matrix. The architecture of the MAD portfolio optimization tools references the scenarios through a function handle so scenarios that have been set cannot be accessed directly as a property of the PortfolioMAD object.

Setting Scenarios Using the PortfolioMAD Function

Suppose that you have a matrix of scenarios in the AssetScenarios variable. The scenarios are set through the PortfolioMAD object with:

m = [ 0.05; 0.1; 0.12; 0.18 ];
C = [ 0.0064 0.00408 0.00192 0;
0.00408 0.0289 0.0204 0.0119;
0.00192 0.0204 0.0576 0.0336;
0 0.0119 0.0336 0.1225 ];

m = m/12;
C = C/12;

AssetScenarios = mvnrnd(m, C, 20000);

p = PortfolioMAD('Scenarios', AssetScenarios);

disp(p.NumAssets);
disp(p.NumScenarios);
4

20000

Notice that the PortfolioMAD object determines and fixes the number of assets in NumAssets and the number of scenarios in NumScenarios based on the scenario’s matrix. You can change the number of scenarios by calling the PortfolioMAD object with a different scenario matrix. However, once the NumAssets property has been set in the object, you cannot enter a scenario matrix with a different number of assets. The getScenarios function lets you recover scenarios from a PortfolioMAD object. You can also obtain the mean and covariance of your scenarios using estimateScenarioMoments.

Although not recommended for the casual user, an alternative way exists to recover scenarios by working with the function handle that points to scenarios in the PortfolioMAD object. To access some or all the scenarios from a PortfolioMAD object, the hidden property localScenarioHandle is a function handle that points to a function to obtain scenarios that have already been set. To get scenarios directly from a PortfolioMAD object p, use

scenarios = p.localScenarioHandle([], []);
and to obtain a subset of scenarios from rows startrow to endrow, use
scenarios = p.localScenarioHandle(startrow, endrow);
where 1startrowendrownumScenarios.

Setting Scenarios Using the setScenarios Function

You can also set scenarios using setScenarios. For example, given the mean and covariance of asset returns in the variables m and C, the asset moment properties can be set:

m = [ 0.05; 0.1; 0.12; 0.18 ];
C = [ 0.0064 0.00408 0.00192 0;
0.00408 0.0289 0.0204 0.0119;
0.00192 0.0204 0.0576 0.0336;
0 0.0119 0.0336 0.1225 ];

m = m/12;
C = C/12;

AssetScenarios = mvnrnd(m, C, 20000);

p = PortfolioMAD;
p = setScenarios(p, AssetScenarios);

disp(p.NumAssets);
disp(p.NumScenarios);
4

20000

Estimating the Mean and Covariance of Scenarios

The estimateScenarioMoments function obtains estimates for the mean and covariance of scenarios in a PortfolioMAD object.

m = [ 0.05; 0.1; 0.12; 0.18 ];
C = [ 0.0064 0.00408 0.00192 0;
0.00408 0.0289 0.0204 0.0119;
0.00192 0.0204 0.0576 0.0336;
0 0.0119 0.0336 0.1225 ];

m = m/12;
C = C/12;

AssetScenarios = mvnrnd(m, C, 20000);

p = PortfolioMAD;
p = setScenarios(p, AssetScenarios);
[mean, covar] = estimateScenarioMoments(p)
mean =

    0.0044
    0.0084
    0.0108
    0.0155


covar =

    0.0005    0.0003    0.0002   -0.0000
    0.0003    0.0024    0.0017    0.0010
    0.0002    0.0017    0.0047    0.0028
   -0.0000    0.0010    0.0028    0.0103

Simulating Normal Scenarios

As a convenience, the two functions (simulateNormalScenariosByData and simulateNormalScenariosByMoments) exist to simulate scenarios from data or moments under an assumption that they are distributed as multivariate normal random asset returns.

Simulating Normal Scenarios from Returns or Prices

Given either return or price data, use the simulateNormalScenariosByData function to simulate multivariate normal scenarios. Either returns or prices are stored as matrices with samples going down the rows and assets going across the columns. In addition, returns or prices can be stored in a table or timetable (see Simulating Normal Scenarios from Time Series Data). To illustrate using simulateNormalScenariosByData, generate random samples of 120 observations of asset returns for four assets from the mean and covariance of asset returns in the variables m and C with portsim. The default behavior of portsim creates simulated data with estimated mean and covariance identical to the input moments m and C. In addition to a return series created by portsim in the variable X, a price series is created in the variable Y:

m = [ 0.0042; 0.0083; 0.01; 0.15 ];
C = [ 0.005333 0.00034 0.00016 0;
0.00034 0.002408 0.0017 0.000992;
0.00016 0.0017 0.0048 0.0028;
0 0.000992 0.0028 0.010208 ];

X = portsim(m', C, 120);
Y = ret2tick(X);

Note

Portfolio optimization requires that you use total returns and not just price returns. So, “returns” should be total returns and “prices” should be total return prices.

Given asset returns and prices in variables X and Y from above, this sequence of examples demonstrates equivalent ways to simulate multivariate normal scenarios for the PortfolioMAD object. Assume a PortfolioMAD object created in p that uses the asset returns in X uses simulateNormalScenariosByData:

p = PortfolioMAD;
p = simulateNormalScenariosByData(p, X, 20000);

[passetmean, passetcovar] = estimateScenarioMoments(p)
passetmean =

    0.0033
    0.0085
    0.0095
    0.1503


passetcovar =

    0.0055    0.0004    0.0002    0.0001
    0.0004    0.0024    0.0017    0.0010
    0.0002    0.0017    0.0049    0.0028
    0.0001    0.0010    0.0028    0.0102
The moments that you obtain from this simulation will likely differ from the moments listed here because the scenarios are random samples from the estimated multivariate normal probability distribution of the input returns X.

The default behavior of simulateNormalScenariosByData is to work with asset returns. If, instead, you have asset prices as in the variable Y, simulateNormalScenariosByData accepts a name-value pair argument name 'DataFormat' with a corresponding value set to 'prices' to indicate that the input to the function is in the form of asset prices and not returns (the default value for the 'DataFormat' argument is 'returns'). This example simulates scenarios with the asset price data in Y for the PortfolioMAD object q:

p = PortfolioMAD;
p = simulateNormalScenariosByData(p, Y, 20000, 'dataformat', 'prices');

[passetmean, passetcovar] = estimateScenarioMoments(p)
passetmean =

    0.0043
    0.0083
    0.0099
    0.1500


passetcovar =

    0.0053    0.0003    0.0001    0.0002
    0.0003    0.0024    0.0017    0.0010
    0.0001    0.0017    0.0047    0.0027
    0.0002    0.0010    0.0027    0.0100

Simulating Normal Scenarios with Missing Data

Often when working with multiple assets, you have missing data indicated by NaN values in your return or price data. Although Multivariate Normal Regression goes into detail about regression with missing data, the simulateNormalScenariosByData function has a name-value pair argument name 'MissingData' that indicates with a Boolean value whether to use the missing data capabilities of Financial Toolbox™. The default value for 'MissingData' is false which removes all samples with NaN values. If, however, 'MissingData' is set to true, simulateNormalScenariosByData uses the ECM algorithm to estimate asset moments. This example shows how this works on price data with missing values:

m = [ 0.0042; 0.0083; 0.01; 0.15 ];
C = [ 0.005333 0.00034 0.00016 0;
0.00034 0.002408 0.0017 0.000992;
0.00016 0.0017 0.0048 0.0028;
0 0.000992 0.0028 0.010208 ];

X = portsim(m', C, 120);
Y = ret2tick(X);
Y(1:20,1) = NaN;
Y(1:12,4) = NaN;

Notice that the prices above in Y have missing values in the first and fourth series.

p = PortfolioMAD;
p = simulateNormalScenariosByData(p, Y, 20000, 'dataformat', 'prices');

q = PortfolioMAD;
q = simulateNormalScenariosByData(q, Y, 20000, 'dataformat', 'prices', 'missingdata', true);

[passetmean, passetcovar] = estimateScenarioMoments(p)
[qassetmean, qassetcovar] = estimateScenarioMoments(q)
passetmean =

    0.0095
    0.0103
    0.0124
    0.1505


passetcovar =

    0.0054    0.0000   -0.0005   -0.0006
    0.0000    0.0021    0.0015    0.0010
   -0.0005    0.0015    0.0046    0.0026
   -0.0006    0.0010    0.0026    0.0100


qassetmean =

    0.0092
    0.0082
    0.0094
    0.1463


qassetcovar =

    0.0071   -0.0000   -0.0006   -0.0006
   -0.0000    0.0032    0.0023    0.0015
   -0.0006    0.0023    0.0064    0.0036
   -0.0006    0.0015    0.0036    0.0133
The first PortfolioMAD object, p, contains scenarios obtained from price data in Y where NaN values are discarded and the second PortfolioMAD object, q, contains scenarios obtained from price data in Y that accommodate missing values. Each time you run this example, you get different estimates for the moments in p and q.

Simulating Normal Scenarios from Time Series Data

The simulateNormalScenariosByData function accepts asset returns or prices stored in table or timetable. The simulateNormalScenariosByData function implicitly works with matrices of data or data in a table or timetable object using the same rules for whether the data are returns or prices. To illustrate, use array2timetable to create a timetable for 14 assets from CAPMuniverse and the use the timetable to simulate scenarios for PortfolioCVaR.

load CAPMuniverse
time = datetime(Dates,'ConvertFrom','datenum');
stockTT = array2timetable(Data,'RowTimes',time, 'VariableNames', Assets);
stockTT.Properties
% Notice that GOOG has missing data, because it was not listed before Aug 2004
head(stockTT, 5);
ans = 

  struct with fields:

             Description: ''
                UserData: []
          DimensionNames: {'Time'  'Variables'}
           VariableNames: {'AAPL'  'AMZN'  'CSCO'  'DELL'  'EBAY'  'GOOG'  'HPQ'  'IBM'  'INTC'  'MSFT'  'ORCL'  'YHOO'  'MARKET'  'CASH'}
    VariableDescriptions: {}
           VariableUnits: {}
      VariableContinuity: []
                RowTimes: [1471×1 datetime]

Use the 'MissingData' option offered by PortfolioMAD to account for the missing data.

p = PortfolioMAD;
p = simulateNormalScenariosByData(p, stockTT, 20000 ,'missingdata',true);
[passetmean, passetcovar] = estimateScenarioMoments(p)
passetmean =

    0.0017
    0.0013
    0.0005
    0.0001
    0.0019
    0.0049
    0.0003
    0.0003
    0.0006
   -0.0001
    0.0005
    0.0011
    0.0002
    0.0001


passetcovar =

    0.0014    0.0005    0.0006    0.0006    0.0006    0.0003    0.0005    0.0003    0.0006    0.0004    0.0005    0.0007    0.0002   -0.0000
    0.0005    0.0025    0.0007    0.0005    0.0010    0.0005    0.0005    0.0003    0.0006    0.0004    0.0006    0.0012    0.0002   -0.0000
    0.0006    0.0007    0.0013    0.0006    0.0007    0.0004    0.0006    0.0004    0.0008    0.0005    0.0008    0.0008    0.0002   -0.0000
    0.0006    0.0005    0.0006    0.0009    0.0006    0.0002    0.0005    0.0003    0.0006    0.0004    0.0005    0.0006    0.0002   -0.0000
    0.0006    0.0010    0.0007    0.0006    0.0018    0.0007    0.0005    0.0003    0.0006    0.0005    0.0007    0.0011    0.0002    0.0000
    0.0003    0.0005    0.0004    0.0002    0.0007    0.0013    0.0002    0.0002    0.0002    0.0002    0.0003    0.0011    0.0001   -0.0000
    0.0005    0.0005    0.0006    0.0005    0.0005    0.0002    0.0010    0.0003    0.0005    0.0003    0.0005    0.0006    0.0002   -0.0000
    0.0003    0.0003    0.0004    0.0003    0.0003    0.0002    0.0003    0.0005    0.0004    0.0002    0.0004    0.0004    0.0002   -0.0000
    0.0006    0.0006    0.0008    0.0006    0.0006    0.0002    0.0005    0.0004    0.0011    0.0005    0.0007    0.0007    0.0002   -0.0000
    0.0004    0.0004    0.0005    0.0004    0.0005    0.0002    0.0003    0.0002    0.0005    0.0006    0.0004    0.0005    0.0002   -0.0000
    0.0005    0.0006    0.0008    0.0005    0.0007    0.0003    0.0005    0.0004    0.0007    0.0004    0.0014    0.0008    0.0002   -0.0000
    0.0007    0.0012    0.0008    0.0006    0.0011    0.0011    0.0006    0.0004    0.0007    0.0005    0.0008    0.0020    0.0002   -0.0000
    0.0002    0.0002    0.0002    0.0002    0.0002    0.0001    0.0002    0.0002    0.0002    0.0002    0.0002    0.0002    0.0001   -0.0000
   -0.0000   -0.0000   -0.0000   -0.0000    0.0000   -0.0000   -0.0000   -0.0000   -0.0000   -0.0000   -0.0000   -0.0000   -0.0000    0.0000

Use the name-value input 'DataFormat' to handle return or price data and 'MissingData' to ignore or use samples with missing values. In addition, simulateNormalScenariosByData extracts asset names or identifiers from a table or timetable if the argument 'GetAssetList' is set to true (the default value is false). If the 'GetAssetList' value is true, the identifiers are used to set the AssetList property of the PortfolioMAD object. Thus, repeating the formation of the PortfolioMAD object p from the previous example with the 'GetAssetList' flag set to true extracts the column names from the timetable object:

p = simulateNormalScenariosByData(p, stockTT, 20000 ,'missingdata',true, 'GetAssetList', true);
disp(p.AssetList)
 'AAPL'    'AMZN'    'CSCO'    'DELL'    'EBAY'    'GOOG'    'HPQ'    'IBM'    'INTC'    'MSFT'    'ORCL'    'YHOO'    'MARKET'    'CASH'

If you set the'GetAssetList' flag set to true and your input data is in a matrix, simulateNormalScenariosByData uses the default labeling scheme from setAssetList as described in Setting Up a List of Asset Identifiers.

Simulating Normal Scenarios for Mean and Covariance

Given the mean and covariance of asset returns, use the simulateNormalScenariosByMoments function to simulate multivariate normal scenarios. The mean can be either a row or column vector and the covariance matrix must be a symmetric positive-semidefinite matrix. Various rules for scalar expansion apply. To illustrate using simulateNormalScenariosByMoments, start with moments in m and C and generate 20,000 scenarios:

m = [ 0.0042; 0.0083; 0.01; 0.15 ];
C = [ 0.005333 0.00034 0.00016 0;
0.00034 0.002408 0.0017 0.000992;
0.00016 0.0017 0.0048 0.0028;
0 0.000992 0.0028 0.010208 ];

p = PortfolioMAD;
p = simulateNormalScenariosByMoments(p, m, C, 20000);
[passetmean, passetcovar] = estimateScenarioMoments(p)
passetmean =

    0.0040
    0.0084
    0.0105
    0.1513


passetcovar =

    0.0053    0.0003    0.0002    0.0001
    0.0003    0.0024    0.0017    0.0009
    0.0002    0.0017    0.0048    0.0028
    0.0001    0.0009    0.0028    0.0102

simulateNormalScenariosByMoments performs scalar expansion on arguments for the moments of asset returns. If NumAssets has not already been set, a scalar argument is interpreted as a scalar with NumAssets set to 1. simulateNormalScenariosByMoments provides an additional optional argument to specify the number of assets so that scalar expansion works with the correct number of assets. In addition, if either a scalar or vector is input for the covariance of asset returns, a diagonal matrix is formed such that a scalar expands along the diagonal and a vector becomes the diagonal.

See Also

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