AR(p)Model
q-order
truncation
truncation MR(q)Model
trailing tail trailing tail
ARMA(p,q)
Model
Second, the grid search method. Combining the
AIC or BIC criterion, traverse different parameter
combinations (p, d, q) and select the optimal model
that minimizes the information criterion.
Third, residual test. The Ljung-Box test is
performed on the model residuals to ensure that they
conform to the white noise characteristics and to
avoid under-extraction of information. The Ljung-
Box test's main objective is to determine if a time
series has substantial autocorrelation by summing the
series' autocorrelation coefficients. A chi-square
distribution with degrees of freedom m is obeyed by
its test statistic, Q. The original hypothesis of the test
(H0) is that the individual values of the time series are
independent (i.e., there is no autocorrelation) and the
series is white noise; the alternative hypothesis (H1)
is that the individual values of the time series are not
independent and that there is autocorrelation. The test
begins with the selection of the appropriate lag order,
and the choice of the end of the lag is usually based
on the sample size and analytical needs. Generally,
the lag order can be set to 1/4 of the length of the
series. Second, calculate the test statistic. The Ljung-
Box statistic Q, which measures the autocorrelation
of the series over multiple lags, is calculated
according to the formula.The initial hypothesis is
disproved and the series is deemed autocorrelated if
the computed Q value translates into a p-value below
the significance level, which is typically 0.05.
3 EMPIRICAL ANALYSIS
3.1 Data Selection
Studying the trajectory of the closing price of
NVIDIA's stock is crucial because of the company's
representative position in the semiconductor sector on
the US stock exchange. In this paper, the closing price
of NVIDIA stock from January 1, 2022, to January
31, 2025, is selected as the original data (a total of
751) for time series analysis(Qi, 2021).
3.2 Smoothness Test and Difference
Processing
This study examines the smoothness of 751 closing
price data points from NVIDIA's trading days during
the previous three years and the smoothness of the
raw data is observed by plotting the ACF chart and
PACF chart, and the results are shown in Figures 1
and 2 below.
The autocorrelation coefficient in the ACF plot
(Figure 1) is 1 at lag 0, which is due to the perfect
correlation of any series with itself. As the lag
increases, the autocorrelation coefficient decreases
slightly but still remains at a high level. This indicates
that NVIDIA stock price has significant
autocorrelation in the short or long run.
However, in the PACF plot (Figure 2), the biased
autocorrelation coefficient is 1 at lag 0; at lag 1,there
is considerable autocorrelation at lag 1, as indicated
by the biased autocorrelation coefficient being
significantly larger than 0. As the lag period
increases, the partial autocorrelation coefficient
decreases rapidly and approaches 0 after lag 2. This
indicates that there is significant autocorrelation in
NVIDIA's stock price in the short term (lag 1), but no
significant autocorrelation in the longer lag period.
In summary, Figure 1 and Figure 2 show that
NVIDIA stock price has significant autocorrelation in
the short term (lag 1) and no significant
autocorrelation in the longer lag. Thus, this property
suggests that the NVIDIA stock price data has some
predictability in the short run, but exhibits
randomness over longer periods of time, and the
series is considered to be unstable.