Modelling volatility - ARCH and GARCH models

Transcription

1 Modelling volatility - ARCH and GARCH models Beáta Stehlíková Time series analysis Modelling volatility- ARCH and GARCH models p.1/33

2 Stock prices Weekly stock prices (library quantmod) Continuous returns: At the beginning of the term we analyzed their autocorrelations in a HW Modelling volatility- ARCH and GARCH models p.2/33

3 Returns Time evolution: Modelling volatility- ARCH and GARCH models p.3/33

4 Returns Based on ACF, they look like a white noise: Modelling volatility- ARCH and GARCH models p.4/33

5 Returns We model them as a white noise: residuals are just - up to a contant - the returns If the absolute value of a residual is small, usually follows a residual with a small absolute value Similarly, after a residual with a large absolute value, there is often another residual with a large absolute value - it can be positive or negative, so it cannot be seen on the ACF Second powers will likely be correlated (but this does not hold for a white noise) Modelling volatility- ARCH and GARCH models p.5/33

6 Returns ACF of squared residuals: significant autocorrelation QUESTION: Which model can capture this property? Modelling volatility- ARCH and GARCH models p.6/33

7 Returns Possible explanation: nonconstant variance Modelling volatility- ARCH and GARCH models p.7/33

8 ARCH and GARCH models u is not a white noise, but u t = σ 2 t η t, where η is a white noise with unit variance, i.e., u t N(0,σ 2 t ) ARCH model (autoregressive conditional heteroskedasticity) - equation for variance σ 2 t : σ 2 t= ω+α 1 u 2 t 1+...α q u 2 t q Constraints on parameters: variance has to be positive: ω >0,α 1,...,α q 1 0,α q >0 stationarity: α α q <1 Modelling volatility- ARCH and GARCH models p.8/33

9 ARCH and GARCH models Disadvantages of ARCH models: a small number of terms u 2 t i is often not sufficient - squares of residuals are still often correlated for a larger number of terms, these are often not significant or the constraints on paramters are not satisfied Generalization: GARCH models - solve these problems Modelling volatility- ARCH and GARCH models p.9/33

10 ARCH and GARCH models GARCH(p,q) model (generalized autoregressive conditional heteroskedasticity) - equation for variance σ 2 t : σ 2 t = ω+α 1 u 2 t α q u 2 t q +β 1 σ 2 t β p σ 2 t p Constraints on parameters: variance has to be positive: ω >0,α 1,...,α q 1 0,α q >0 β 1,...,β p 1 0,β p >0 stationarity: (α α q )+(β β p ) <1 A popular model is GARCH(1,1). Modelling volatility- ARCH and GARCH models p.10/33

11 GARCH models in R Modelling YHOO returns - continued In R: library fgarch function garchfit, model is writen for example like arma(1,1)+garch(1,1) parameter trace=false - we do not want the details about optimization process We have a model constant + noise; we try to model the noise by ARCH/GARCH models Modelling volatility- ARCH and GARCH models p.11/33

12 ARCH(1) Estimation of ARCH(1) model: We check 1. ACF of standardized residuals 2. ACF of squared standardized residuals 3. summary with tests about standardized residuals and their squares Modelling volatility- ARCH and GARCH models p.12/33

13 GARCH models in v R Useful - fitted - - estimated - estimated standard deviation Standardized residuals - residuals divided by their standard deviation rezíduá vydelené ich štadardnou - should be a white noise Also their squares should be a white noise Modelling volatility- ARCH and GARCH models p.13/33

14 ARCH(1) Residuals: Modelling volatility- ARCH and GARCH models p.14/33

15 ARCH(1) Squares: Modelling volatility- ARCH and GARCH models p.15/33

16 Tests about residuals Tests: We have: normality test, Ljung-Box for standardized residuals and their sqaures What is new: testing homoskedasticity for the residuals Modelling volatility- ARCH and GARCH models p.16/33

17 ARCH(2) We try ARCH(2) - results of the tests: Modelling volatility- ARCH and GARCH models p.17/33

18 ARCH(3) ARCH(3) - results of the tests: Modelling volatility- ARCH and GARCH models p.18/33

19 ARCH(4) ARCH(4) - results of the tests: No autocorrelation in residuals and their squares. Modelling volatility- ARCH and GARCH models p.19/33

20 ARCH(4) ACF of squared residuals: without significant correlation Modelling volatility- ARCH and GARCH models p.20/33

21 ARCH(4) But ARCH coefficients α i are not significant: Modelling volatility- ARCH and GARCH models p.21/33

22 GARCH(1,1) We try GARCH(1,1) Tests: Modelling volatility- ARCH and GARCH models p.22/33

23 GARCH(1,1) Estimates: Modelling volatility- ARCH and GARCH models p.23/33

24 Estimated standard deviation We obtain it : Modelling volatility- ARCH and GARCH models p.24/33

25 Estimated standard deviation Another access to the graphs - plot(model11): Modelling volatility- ARCH and GARCH models p.25/33

26 Predictions We use the function predict with parameter n.ahead (number of observations) Modelling volatility- ARCH and GARCH models p.26/33

27 Predictions Parameter nx - we can change the number of observations from the data which are shown in the plot (here nx=100: Modelling volatility- ARCH and GARCH models p.27/33

28 Predictions Predicted standard deviation: plot(ts(predictions[3])) Modelling volatility- ARCH and GARCH models p.28/33

29 Predictions For a longer time (exercise: compute its limit): Modelling volatility- ARCH and GARCH models p.29/33

30 Application: Value at risk (VaR) Value at risk (VaR) is basicly a quantile Let X be a portfolio value, then for example for α=0.05 P(X VaR)=α, A standard GARCH assumes normal distribution - we can compute quantiles Shortcomings: normality assumptions there are also better risk measures than VaR Modelling volatility- ARCH and GARCH models p.30/33

31 Apication: Value at risk (VaR) WHAT WE WILL DO: Start with N observations of returns Estimate the GARCH model. Make a prediction for standard deviation and using the prediction we construct VaR for returns for the following day Every day move the window with data (we have a new observation), estimate GARCH again and compute thes new VaR Modelling volatility- ARCH and GARCH models p.31/33

32 Not required - for those interested trading-using-garch-volatility-forecast/ "... Now, let s create a strategy that switches between mean-reversion and trend-following strategies based on GARCH(1,1) volatility forecast." + R code From the website: Modelling volatility- ARCH and GARCH models p.32/33

33 Other models for volatility Threshold GARCH: u t >0-"good news", u t <0-"bad news" TARCH can model their different effect on volatility leverage effect: bad news have a higher impact We do not model variance (as in ARCH/GARCH models), but its logarithm exponential GARCH any power of standard deviation power GARCH and others... Modelling volatility- ARCH and GARCH models p.33/33