Imperial College Business School, London June 25, 2010
Introduction Contents Types of Volatility Predictive Density Risk Neutral Density Forecasting Empirical Volatility Density Forecasting Density model averaging Bayesian Approximation using Ranking Measures Thick Model Averaging
Data Contents Results Risk Neutral Volatility Density Forecasting Risk Neutral to Objective Volatility Density Forecasting Real World Volatility Density Forecasting Investment Strategy Results - Investment Strategy Conclusions
Introduction Forecasting volatility of a financial instrument has been the center of scientific research for almost three decades. If we allow more attributes to play a roll in the forecasting density then predicting directly that density and not characteristics of it becomes the imminent task to perform. Why density forecasting? Complicated structure derivative products need to deviate from log normal distributional assumption of the underlying. Cutting edge portfolio optimization strategies. Strategic Risk Management. point or interval estimation are inadequate tools for those responsible for decision making.
Types of Volatility Predictive Density Two distinct schools of thought addressing the problem of volatility density forecasting Real World Density Forecasting. Reflects the dynamics of real prices. Risk Neutral Density Forecasting. Predictive densities are obtained from a set of option prices.
Types of Volatility Predictive Density cont. Implied Volatility Density Forecasting. Breeden & Litzenberger (1978) established a relationship between the second derivative of the option price with respect to the strike price and the risk neutral density. Empirical Volatility Density Forecasting. GARCH-type Bootstrap related methods
How devastating a wrong model selection could be to the final result? Model misspecification risk jeopardizes the forecasting performance. Solution to the problem is to use Model Averaging Techniques. Two methods: Thick Model Averaging (Granger and Jeon, 2004). Bayesian Approximation.
Risk Neutral Density Forecasting Ross (1976) and Cox and Ross (1976) the call price equals: c (S T ) = e rt K S T f Q t (S T ) ds T e rt Kp Q Breeden and Litzenberger: ft Q (S T ) = e rt 2 c t (S T, K, T, t) K 2 K=ST
Risk Neutral Density Forecasting cont. To approximate the above density function, ft Q to formulate a butterfly spread: (S T ), we have e rt 2c (S t, K, T, t) + c (S t, K s, T, t) + c (S t, K + s, T, t) s 2 P (S t, K, T, t) lim s 0 s 2 e rt ft Q (S T )
Risk Neutral Density Forecasting cont. From the above is apparent the need to have a reliable OPTION PRICING MODEL. The literature in inundated with various suggestions. Of the most important: The analytic pricing formula of Grunbichler and Longstaff a Square Root Mean Reverting Process. dv T = (α κv T ) dt + σ V T dz T
Risk Neutral Density Forecasting cont. Based on Feller (1951) and Cox and Ross (1985) they proved that the risk adjusted underlying asset is noncentrally χ 2 (ν, λ) distributed. This gives the analytical solution to the PDE of the option pricing equation which is the following: c (V T, K) = D T E Q [ (V T K) +] [ c (V t, K) = e rt e βt V t Q (γk ν + 4, λ) + α ( β 1 e βt ) Q (γk ν + 2, λ) e rt KQ (γk ν, λ) ]
Risk Neutral Density Forecasting cont. The Log Mean Reverting Gaussian Process model of Detemple and Osakwe: d ln V T = (α λ ln (V T )) dt + σdz T V T = e rt V φt + η2 λ 2 N (δ + η) KN (δ) 0 e α(1 φ t )
Risk Neutral Density Forecasting cont. The risk adjusted Jump Diffusion Mean Reverting model of Clewlow and Strickland log V t = ( α (µ r log V t ) 1 2 σ2) t + σ tε 1t + (µ jump + σ jump ε 2t κµ jump t) (u t > κ t) ε 1i, ε 2i N { (0, 1) 1 ui > κ t u i U (0, 1) where u i 0 otherwise
Risk Neutral Density Forecasting cont. In a spirit similar to Schwartz (1997) (non constant volatility component for the equation that describes the underlying) as well as Detemple and Osakwe (2000) (log Ornstein Uhlenbeck model) we employ a Stochastic Volatility Double Mean Reverting process (both the underlying and the volatility process mean revert). d log V t = α (µ r log V t ) dt + σ t dw 1t d log σ 2 t = η ( ϑ log σ 2 t ) dt + γdw2t Barndorff Nielsen and Shephard model (2001) alternatives. Capture the driving dynamics of volatility options using a Gamma Ornstein Uhlenbeck process with stochastic time change.
Risk Neutral Density Forecasting cont. dy t = λy t dt + dz λt The process here represents the volatility of the underlying and can be simulated as: y tn = (1 λ t) y tn 1 + N tn κ=n tn 1 +1 x κ e λ tuκ pricing equation equals, log V t = ( α (µ r log V t ) 1 2 σ2 t ) t + σ t tε1t
Risk Neutral Density Forecasting cont. Central Tendency (time dependent mean) with Jumps µ t = c + bµ t 1 + δε t c = θ ( 1 e α t) b = e α t The underlying follows OU jump process with time dependent mean: log V t = ( α (µ t r log V t ) 1 2 σ2) t + σ tε 1t + (µ jump + σ jump ε 2t κµ jump t) (u t > κ t)
Transforming Risk Neutral to Real World Density Ait-Sahalia & Lo (2000) The stochastic discount factor for all options is a random variable that equals: ζ (S T ) = e r(t t) f Q (S T ) f P (S T ) = λu (S T ) U (S t ) The relative risk aversion equals, RRA = x ζ t (x) ζ t (x) U (x) = x U (x)
Transforming Risk Neutral to Real World Density cont. Power utility assumption here: { x 1 γ U (x) = 1 γ γ 1 log (x) γ = 1 Then the Relative Risk Aversion equals: RRA = x U (x) γx γ 1 = x U (x) x γ = γ f P (x) = 0 x γ f Q (x) f Q (y) y γ dy
Transforming Risk Neutral to Real World Density cont. In steps the procedure is described as follows: 1. Select an option pricing model. To avoid problems related with model uncertainty later we will use model averaging procedures instead of making such a decision. 2. Calibrate the parameters of the model using observed option prices. 3. Using a butterfly spread to approximate the 2nd derivative of the price with respect to strike obtain the empirical risk neutral predictive distribution of the underlying (the VIX index) for a month ahead.
Transforming Risk Neutral to Real World Density cont. 4. Obtain random draws from the empirical predictive density in order to calibrate the Gamma and the log-normal distribution that we want to use in addition to the empirical. 5. Transform the risk neutral distribution (either the empirical, Gamma or log-normal) to the objective one according to Bliss and Panigirtzoglou method i.e. connect the investors risk preferences (using the power utility function for example) with the risk neutral real world density. 6. The risk premium is obtained by maximizing the Berkowitz s test p-value related with the realizations of the index with respect to the transformed predictive distribution. 7. Repeat steps 1 to 4 for the whole sequence of forecasts.
Empirical Volatility Density Forecasting To account for some of the most widely accepted characteristics of conditional volatility the model that have been used in the analysis are: Model Variance Equation GARCH p q h t = ω + a i ε 2 t i + β j h t j Absolute Value GARCH ht = ω + GJR h t = ω + i=1 j=1 p a i ε t i c + i=1 q β j ht j j=1 p α i ε 2 t i + γε2 t i I {ε t i < 0} + i=1 q β j h t j j=1
Empirical Volatility Density Forecasting cont. TGARCH h t = ω + p a i ε t i c + i=1 q β j ht j j=1 Asymetric Power GARCH ( δ ) 2 p ht j = ω + a i ε t i δ γ ε t i δ I { ε t i < 0 } i=1 + q ( ) δ β j ht j j=1
Empirical Volatility Density Forecasting cont. The error parameters are allowed to be distributed less restrictively: Normal Distribution Generalized Normal Distribution Student s-t distribution Skew-t distribution To simulate a sufficient large number of variance forecasts for time t we take n independent random draws from the error distribution and add them to the volatility equation.
Empirical Volatility Density Forecasting cont. For each draw z t p,i i = 1, 2..., n we estimate ε t p,i i = 1, 2..., n. Next forecast h t,i i = 1, 2..., n. At the end we have a volatility density estimate for time t for every univariate conditional volatility model with lags (p, q) and marginal distribution the same one as that of the error distribution.
Empirical Volatility Density Forecasting cont. In steps the procedure is described as follows: 1. Select a model to forecast volatility and select an error distribution to use for that model. 2. Using return data until t p (p being the lag value) we calibrate volatility parameters using maximum likelihood. Notice that depending on the error distribution the log likelihood considered necessary to calibrate that volatility model should adjust accordingly. 3. Take n random draws from that selected error density to simulate values for z t p.
Empirical Volatility Density Forecasting cont. 4. Estimate ε t p,i = h t p,i z t p,i i = 1, 2..., n 5. Create n independent volatility forecasts for each time step using the simulated error values. 6. Fit the volatility sample to a choice of three distributions. First to the empirical distribution using a kernel regression, second to the Gamma and third to the log-normal distribution. 7. Asses the accuracy of each model using the Berkowitz test to the whole sequence of forecasts. The model that generates the highest p-value of the test is the optimal model to forecast the volatility density of the index.
Density model averaging Bayesian Model Averaging Bayesian Inference Let y = {y 1, y 2,... y n } iid observations with θ unknown parameters and prior distribution π (θ η). Inferences for θ can be made based on the posterior distribution p(θ y, η) = p(y,θ η) p(y η) = p(y,θ η) p(y,θ η)dϑ = p(y θ)p(θ η) p(y θ)p(θ η)dϑ = l(y θ)p(θ) m(y η) MCMC algorithm used to simulate posterior parameters distributions through sampling from a Marcov Chain that has as its unique stationary distribution the probability distribution of interest.
Bayesian Approximation using Ranking Measures The estimation of the marginal distribution difficult to perform. Kass & Wasserman (1995) used the Schwartz Information Criterion to approximate the Bayes Factor: log BF ij L Mi L Mj + k M i k Mj 2 log p
Bayesian Approximation using Ranking Measures cont. To perform averaging: Compute for each model available its Ranking Measure (BIC, AIC, MSE etc). The approximated marginal distribution equals: m = RM Mi Calculate posterior probabilities by dividing the approximated marginal distribution of each model with the sum of marginals of all models.
Thick Model Averaging Simple Thick Model Average (Granger and Jeon, 2003): Rank models according to an error criterion (eg BIC, AIC, Pesaran, Zaffaroni, 2006). Select a percentage of them (most commonly used is 10% to 50% of best performing models) and average them.
Risk Neutral Volatility Density Averaging Bayesian Approximation A collection of alternative competing pricing proposal is used. Each period these models are fitted to the real option data and the Mean Squared Error of that fit is reported as the first RM. ( ) N C n,t 2 N C n,t en,t 2 n=1 n=1 MSE t = = N N
Bayesian Approximation cont. Additional Ranking Measures are the Akaike and Bayesian Information Criteria. AIC t = e 2k N t BIC t = N k N 1 N N n=1 1 N n=1 en,t 2 = e 2k N t MSE t N en,t 2 = N k N MSE t
Bayesian Approximation cont. All models will be weighted according to their Bayes Factors. BF Mi,t BF Mi,t BF Mi,t BIC M i,t n BIC Mi,t i=1 AIC M i,t n AIC Mi,t i=1 MSE M i,t n MSE Mi,t i=1 i = 1,..., n i = 1,..., n i = 1,..., n
Thick Model Averaging Uses only a percentage of the best competing models. w Mi,t = w Mi,t = w Mi,t = p BIC Mi,t i=1 p p AIC Mi,t i=1 p n MSE Mi,t i=1 p i = 1,..., p i = 1,..., p i = 1,..., p p is the number of models included in the averaging scheme depending on the keeping percentage here p = 33%n or p = 50%n
Empirical Volatility Density Averaging Bayesian Approximation Estimate GARCH-type models using the maximum likelihood. Calculate relevant Ranking Criteria. ( ) N h Mi,t 2 h Mi,t i=1 MSE Mt = = N N em 2 i,t i=1 N i = 1,..., n AIC Mt = 2L Mi,t + 2 N k M i,t i = 1,..., n BIC Mt = 2L Mi,t + log(n) N k M i,t i = 1,..., n
Data - Risk Neutral Volatility Density Forecasting The VIX index from 6/2010 until 10/2009. 44 surfaces - monthly frequency. Every month a new option surface was acquired and based on that a forecast for the next month s implied volatility density was made. 32 surfaces were discharged from the sample to calibrate the Risk Premium (following Bliss and Panigirtzoglou). A rolling window was used to recalibrate the risk premium every month. The last 12 remaining surfaces were used for density forecasting. Using the option prices of every month, forecasts for the implied volatility density of next month were made.
Data - Risk Neutral Volatility Density Forecasting cont. Filtering was applied to the option prices so that there were disregarded: Options with small maturity (less than ten days). Options with more than 365 days maturity. Options with value smaller than $0.05. Options that had zero volume.
Data - Empirical Volatility Density Forecasting The S&P 500 index from 3/1950 to 10/2009. 716 data points - monthly frequency. 12 realizations constitute the out of sample period. Actual volatility: high-low estimator (Bollen and Inder, 2002) ĥ = (ln P H t ln P Lt ) 2 4 ln 2
Results - Risk Neutral Volatility Density Forecasting Berkowitz test was used (Berkowitz, 2001). The additional advantage of this indicator is that both independence and uniformity are jointly tested with this test. The optimal model is the one that produced the highest test p-value.
Results - Risk Neutral Volatility Density Forecasting cont. Rank Model P-value Berkowitz Rank Model P-value Berkowitz 1 TM-MSE 33% Log-Normal 0.7021 19 TM-MSE 50% Gamma 0.6074 2 TM-AIC 33% Log-Normal 0.6984 20 TM-BIC 33% Emprirical 0.5981 3 TM-BIC 50% Log-Normal 0.6897 21 BA-MSE Gamma 0.5841 4 TM-MSE 50% Log-Normal 0.6840 22 TM-BIC 33% Gamma 0.5822 5 TM-AIC 50% Log-Normal 0.6823 23 BA-AIC Gamma 0.5820 6 TM-MSE 50% Emprirical 0.6554 24 BA-BIC Gamma 0.5803 7 TM-BIC 50% Emprirical 0.6542 25 BA-MSE Emprirical 0.5750 8 TM-BIC 33% Log-Normal 0.6520 26 BA-AIC Emprirical 0.5729 9 TM-MSE 33% Emprirical 0.6500 27 BA-BIC Emprirical 0.5711 10 TM-AIC 33% Emprirical 0.6482 28 BEST MSE Log-Normal 0.5496 11 TM-AIC 50% Emprirical 0.6477 29 BEST MSE Emprirical 0.5240 12 BA-MSE Log-Normal 0.6354 30 BEST BIC Log-Normal 0.5105 13 BA-AIC Log-Normal 0.6332 31 BEST AIC Log-Normal 0.5087 14 BA-BIC Log-Normal 0.6313 32 BEST AIC Emprirical 0.4940 15 TM-MSE 33% Gamma 0.6268 33 BEST BIC Emprirical 0.4895 16 TM-AIC 33% Gamma 0.6246 34 BEST MSE Gamma 0.4718 17 TM-BIC 50% Gamma 0.6167 35 BEST BIC Gamma 0.4376 18 TM-AIC 50% Gamma 0.6081 36 BEST AIC Gamma 0.4327
Results - Risk Neutral Volatility Density Forecasting cont. Catholic dominance of the averaging schemes. Top performer is Thick Model Averaging. Assumption of either a log-normal or an empirical distributional pattern for the volatility process is a more preferable option (as opposed to a gamma equivalent). The highest p-values are recorded for option pricing methods that assume a process for the underling that exhibits either jumps (with constant mean and variance) or has a stochastic volatility dynamic (without jumps).
Results - Risk Neutral Volatility Density Forecasting cont. For Best Single Model strategy the log-normal or the empirical density are still more preferable choices than the gamma distribution for the volatility. In total Best Single Proposals fail to solidify their status.
Results - Risk Neutral to Objective Volatility Density Forecasting Rank Model P-value Berkowitz Rank Model P-value Berkowitz 1 TM-AIC 33% Emprirical 0.1047 19 BEST BIC Gamma 0.0770 2 TM-MSE 50% Emprirical 0.0985 20 BEST BIC Log-Normal 0.0720 3 TM-AIC 33% Log-Normal 0.0974 21 BEST BIC Emprirical 0.0741 4 TM-MSE 50% Log-Normal 0.0962 22 TM-AIC 50% Gamma 0.0761 5 TM-BIC 33% Emprirical 0.0910 23 TM-BIC 50% Gamma 0.0759 6 TM-MSE 33% Emprirical 0.0910 24 BEST MSE Gamma 0.0758 7 TM-AIC 50% Emprirical 0.0894 25 BEST MSE Log-Normal 0.0805 8 TM-MSE 33% Log-Normal 0.0890 26 BEST MSE Emprirical 0.0696 9 TM-AIC 50% Log-Normal 0.0880 27 BA-BIC Emprirical 0.0743 10 TM-BIC 50% Log-Normal 0.0877 28 BA-AIC Emprirical 0.0740 11 TM-BIC 50% Emprirical 0.0860 29 BA-MSE Emprirical 0.0737 12 TM-MSE 50% Gamma 0.0851 30 TM-MSE 33% Gamma 0.0728 13 TM-BIC 33% Log-Normal 0.0846 31 BA-BIC Log-Normal 0.0695 14 TM-AIC 33% Gamma 0.0840 32 BA-AIC Log-Normal 0.0690 15 BEST AIC Gamma 0.0803 33 BA-MSE Log-Normal 0.0685 16 BEST AIC Log-Normal 0.0801 34 BA-BIC Gamma 0.0677 17 BEST AIC Emprirical 0.0748 35 BA-AIC Gamma 0.0674 18 TM-BIC 33% Gamma 0.0780 36 BA-MSE Gamma 0.0670
Results - Risk Neutral to Objective Volatility Density Forecasting cont. Averaging schemes still prevail to the single modeling alternatives. Thick Modeling under all different ranking criteria and keeping percentages of the best in sample models dominates again. For Best Single Model strategy the Gamma density is now more preferable (inconsistent performance).
Results - Real World Volatility Density Forecasting Empirical Distribution Log-Normal Gamma Rank Volatility Density Model P-value Volatility Density Model P-value Volatility Density Model P-value 1 TM 40% AIC 0.093 TM 40% AIC 0.103 TM 40% AIC 0.038 2 TM 40% BIC 0.093 TM 40% BIC 0.103 TM 40% BIC 0.038 3 TM 30% AIC 0.084 TM 50% AIC 0.103 TM 50% AIC 0.036 4 TM 30% BIC 0.084 TM 50% BIC 0.103 TM 50% BIC 0.036 5 TM 50% AIC 0.084 TM 30% AIC 0.091 TM 30% AIC 0.002 6 TM 50% BIC 0.084 TM 30% BIC 0.091 TM 30% BIC 0.002 7 TM 20% AIC 0.056 TM 20% BIC 0.055 TM 20% AIC 0.002 8 TM 20% BIC 0.056 TM 20% AIC 0.055 TM 20% BIC 0.002 9 BA AIC 0.000 BA AIC 0.000 TM 10% AIC 0.001 10 BA BIC 0.000 BA BIC 0.000 TM 10% BIC 0.001 11 TM 10% AIC 0.000 TM 10% AIC 0.000 BA AIC 0.000 12 TM 10% BIC 0.000 TM 10% BIC 0.000 BA BIC 0.000 13 BEST AIC 0.000 BEST AIC 0.000 BEST AIC 0.000 14 BEST BIC 0.000 BEST BIC 0.000 BEST BIC 0.000
Results - Real World Volatility Density Forecasting The overwhelming majority of the methods fail to pass the Berkowitz test. Only Thick Model Averaging produces accepts the hypothesis that the predictive density is the true generating process. Log-Normal volatility density produces higher acceptance rates. Best Single Models are still the worst performers.
Investment Strategy Relates only with the option based predictive volatility density. VIX is considered the fear index i.e. an indicator of the movement (not the direction) of the S&P500 index. The forecasted distribution of VIX is used to bet on the equity index.
Investment Strategy cont. Take the current price of VIX. Derive the cumulative probability using the different predictive densities we obtained from the various option models. If the probability surpass a certain threshold we take a short position on the S&P500 index. If it fails to exceed that threshold we take a long position on the index. The threshold is obtained dynamically by optimizing the cumulative wealth of the strategy described above over the last 5 months. at each step the threshold is recalibrated.
Results - Investment Strategy Risk Neutral Volatility Density Forecasting Rank Model cum ret Rank Model cum ret 1 BEST AIC Emprirical 1.284 21 TM-BIC 50% Gamma 1.284 2 BA-AIC Emprirical 1.284 22 BEST MSE Gamma 1.284 3 TM-AIC 33% Emprirical 1.284 23 BA-MSE Gamma 1.284 4 TM-AIC 50% Emprirical 1.284 24 TM-MSE 33% Gamma 1.284 5 BEST BIC Emprirical 1.284 25 TM-MSE 50% Gamma 1.284 6 BA-BIC Emprirical 1.284 26 BEST AIC Log-Normal 1.284 7 TM-BIC 33% Emprirical 1.284 27 BA-AIC Log-Normal 1.284 8 TM-BIC 50% Emprirical 1.284 28 TM-AIC 33% Log-Normal 1.284 9 BEST MSE Emprirical 1.284 29 TM-AIC 50% Log-Normal 1.284 10 BA-MSE Emprirical 1.284 30 BEST BIC Log-Normal 1.284 11 TM-MSE 33% Emprirical 1.284 31 BA-BIC Log-Normal 1.284 12 TM-MSE 50% Emprirical 1.284 32 TM-BIC 33% Log-Normal 1.284 13 BEST AIC Gamma 1.284 33 TM-BIC 50% Log-Normal 1.284 14 BA-AIC Gamma 1.284 34 BEST MSE Log-Normal 1.284 15 TM-AIC 33% Gamma 1.284 35 BA-MSE Log-Normal 1.284 16 TM-AIC 50% Gamma 1.284 36 TM-MSE 33% Log-Normal 1.284 17 BEST BIC Gamma 1.284 37 TM-MSE 50% Log-Normal 1.284 18 BA-BIC Gamma 1.284 38 S&P 500 1.041 19 TM-BIC 33% Gamma 1.284
Results - Investment Strategy Risk Neutral to Objective Volatility Density Forecasting Rank Model cum ret Rank Model cum ret 1 BA-AIC Emprirical 1.812 20 TM-AIC 33% Gamma 1.284 2 BA-BIC Emprirical 1.812 21 TM-AIC 50% Gamma 1.284 3 BA-MSE Emprirical 1.812 22 BEST BIC Gamma 1.284 4 BA-AIC Gamma 1.812 23 TM-BIC 33% Gamma 1.284 5 BA-BIC Gamma 1.812 24 TM-BIC 50% Gamma 1.284 6 BA-MSE Gamma 1.812 25 BEST MSE Gamma 1.284 7 BA-AIC Log-Normal 1.812 26 TM-MSE 33% Gamma 1.284 8 BA-BIC Log-Normal 1.812 27 TM-MSE 50% Gamma 1.284 9 BA-MSE Log-Normal 1.812 28 BEST AIC Log-Normal 1.284 10 BEST AIC Emprirical 1.284 29 TM-AIC 33% Log-Normal 1.284 11 TM-AIC 33% Emprirical 1.284 30 TM-AIC 50% Log-Normal 1.284 12 TM-AIC 50% Emprirical 1.284 31 BEST BIC Log-Normal 1.284 13 BEST BIC Emprirical 1.284 32 TM-BIC 33% Log-Normal 1.284 14 TM-BIC 33% Emprirical 1.284 33 TM-BIC 50% Log-Normal 1.284 15 TM-BIC 50% Emprirical 1.284 34 BEST MSE Log-Normal 1.284 16 BEST MSE Emprirical 1.284 35 TM-MSE 33% Log-Normal 1.284 17 TM-MSE 33% Emprirical 1.284 36 TM-MSE 50% Log-Normal 1.284 18 TM-MSE 50% Emprirical 1.284 37 S&P 500 1.041 19 BEST AIC Gamma 1.284
Results - Investment Strategy Summary Results indicated a higher cumulative wealth under the Bayesian Approximation scheme against all other alternatives. Thick Modeling exhibited equal performance when compared with some single models while the method outperformed other modeling suggestions. All averaging schemes are preferable to investing directly on the S&P500 index. The worst cumulative wealth was generated by single models.
Conclusions Uncertainty of model selection since it is based on the in sample performance according to an error criterion. Out of sample, environment much more volatile thus initial model choice could be rendered suboptimal. Forecasting performance of the best single model out of sample, selected accurately or not in advance, enhanced when attempting averaging. Thick modeling was the best averaging alternative. All in all averaging alternatives outperformed single models. Empirical results as well (investment strategy) substantiated the superiority of averaging schemes. Method has potential for applications not only to volatility density but to any density forecasting aspiration.
Acknowledgements Dr. Paolo Zaffaroni Prof. Nigel Meade ISF Committee