ARCH Effect Explained (Excel)
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Transcript of ARCH Effect Explained (Excel)
ARCH Test tutorial ‐1‐ © Spider Financial Corp, 2012
TheARCHEffectExplainedIn time series and econometric analysis, summary statistics and residual diagnosis often lead us to use a
somewhat mystifying test known as the Auto Regressive Conditional Heteroskedacity (ARCH) effect test,
or ARCH test for short. Why exactly do we use the ARCH test?
You might think of of an ARCH test simply as a means of detecting for the presence of fat tails in the
underlying distribution; but if that were true, what would the test tell us that a test for excess kurtosis
could not?
We need to answer some basic questions about the ARCH test: What does it do? How is it related to the
white‐noise test? When should we utilize what it has to tell us?
Note: For illustration, we are using the IBM stock prices data set for the period between May 17th 1961
and Nov 2nd 1962 (Series B in Box, Jenkins and Reinsell Time series forecast text book (1976)).
BackgroundLet’s assume we have a data set of a univariate { }tx , and we wish to determine whether it exhibits an
ARCH effect.
1. Construct a new time series ty , such that:
2t ty x
2. Form a portmanteau type of test for ty :
1 2
1
: ... 0
: 0o m
k
H
H
ARCH Test tutorial ‐2‐ © Spider Financial Corp, 2012
Where
oH = null hypothesis
1H = alternative hypothesis (auto correlation is significant)
m = maximum number of lags included in the test
i = the population auto correlation function of the squared time series ( ty )
1 k m
In essence, the ARCH effect test is a white‐noise test, but for the squared time series. In other words, we
are investigating a higher order (non‐linear) of auto correlation. How can this information be useful?
The ARCH effect has its roots in time varying conditional volatility, so:
2 2 2 2( )t t t t tE x x E x x
Where
2t = conditional variance
tx =conditional mean
Assuming the time series does not have a significant mean (typical in financial time series), then the
conditional variance is expressed as:
2 2 2 2( )t t t t t tE x x E x E y x
Assuming the squared time series ( ty ) is serially correlated, then conditional volatility ( t ) varies over
time and exhibits a clustering phenomenon (e.g. periods of swings followed by periods of relative calm).
In sum, the ARCH test helps us to detect a time varying phenomenon in the conditional volatility, and
thus suggests different types of models (e.g. ARCH/GARCH) to capture these dynamics.
White‐noise test conditional mean ARMA/ARIMA
ARCH test conditional volatility ARCH/GARCH
Q: Can we have a significant serial correlation in the original time series and a serial correlation in the
squared time series? If so, how can we model that? A: Yes; we use an ARMA‐GARCH mixture model.
ARCH Test tutorial ‐3‐ © Spider Financial Corp, 2012
AnalysisLet’s take a close look at the logarithmic daily returns time series:
The daily return plot suggests a stationary process with zero mean, but the volatility exhibits periods of
relative calm followed by swings (aka volatility clustering).
The white‐noise test identifies insignificant serial correlation in the time series, but the ARCH effect is
significant and indicative of a time‐varying volatility.
1. DailyReturnsDistributionLet’s construct the empirical distribution of the sample time series, and examine the tails in comparison
with those of a Gaussian distribution.
ARCH Test tutorial ‐5‐ © Spider Financial Corp, 2012
The Q‐Q Plot shows an asymmetrical view of the distribution tails; the distribution’s left tail (i.e.
extreme negative returns) are far more deviant from what the Gaussian distribution suggests. This is a
well‐documented phenomenon in the financial time series.
2. CorrelogramAnalysisBy now, we’ve established that the logarithmic daily returns distribution has fat tails (and may be fatter
on the left side than on the right), but where is the time‐varying claim coming from?
In the descriptive statistics table, the ARCH effect suggests a significant serial correlation in the squared
time series. Let’s do the following:
1. Construct the squared time series ( ty ).
2. Run NumXL descriptive statistic wizard, and generate the summary table. The white noise test
on the squared time series is equivalent to the ARCH test on the original time series.
Notice: the ARCH effect for the squared time series is significant, so does this mean we have a
time‐varying kurtosis (fourth moment)? Let’s keep this question in mind, and revisit it in a
separate tutorial!
ARCH Test tutorial ‐6‐ © Spider Financial Corp, 2012
3. Using NumXL Correlogram wizard, generate the ACF/PACF table and plot for the squared time
series.
The ACF/PACF Plot for the squared time series is shown below:
ARCH Test tutorial ‐7‐ © Spider Financial Corp, 2012
3. ARCHModelingUsing the ACF/PACF tables/plot, we can proceed to model the conditional volatility as an ARMA model
(aka GARCH). As in ARMA, we need to identify the AR and MA order of the conditional volatility model.
ConclusionThe ARCH test is a vital tool for examining the time dynamics of the second moments (i.e. conditional
variance). The presence of a significant excess kurtosis is not indicative of a time‐varying volatility, but
the reverse is true: a significant ARCH effect identifies a time‐varying conditional volatility, volatility
clustering (or mean reversion), and, as a result, the presence of a fat‐tailed distribution (i.e. excess
kurtosis > 0).
The correlogram (i.e. ACF/PACF analysis) can be used to identify the order of the AR & MA components
in the GARCH‐type model.
Finally, notice that for financial time series, the negative returns deviate more from normality than
positive ones.