Prediction errors
Under the GARCH model, the variance is driven by the square of the prediction errors \(e = R - \mu\). In order to calculate a GARCH variance, you thus need to first compute the prediction errors. For daily returns, it is common practice to set \(\mu\) equal to the sample average.
You're going to implement this and then verify that there is a large positive autocorrelation in the absolute value of the prediction errors. Positive autocorrelation reflects the presence of volatility clusters. When volatility is above average, it stays above average for some time. When volatility is low, it stays low for some time.
This exercise is part of the course
GARCH Models in R
Exercise instructions
- Set
m
to the mean of the daily S&P 500 returns insp500ret
. - Compute the prediction errors.
- Plot the time series of absolute value of the prediction errors.
- Use the function
acf
to plot the autocorrelation function of the absolute value of the prediction errors.
Hands-on interactive exercise
Have a go at this exercise by completing this sample code.
# Compute the mean daily return
m <- ___(___)
# Define the series of prediction errors
e <- ___ - ___
# Plot the absolute value of the prediction errors
par(mfrow = c(2,1),mar = c(3, 2, 2, 2))
___(___(___))
# Plot the acf of the absolute prediction errors
___