2019). The null hypothesis (H0) of the ADF test is
that the sequence has a unit root; that is, the sequence
is not stationary. The alternative hypothesis (H1) is
that the sequence has no unit root; that is, the
sequence is stationary. Since ADF statistic -3.214 is
less than the critical value of -2.92 at the 5% level,
and the p-value is 0.019 less than 0.05, it rejects the
null hypothesis. Therefore, the data after the third-
order difference is stationary. So it can pass the
stationary test.
3.2 Construction of the ARIMA Model
The formula of the ARIMA model is:
∆
𝑋
=𝛼
𝑋
+𝛼
𝑋
+...+𝛼
𝑋
+
𝜀
+𝛽
𝜀
+𝛽
𝜀
+...+𝛽
𝜀
, (1)
where ∆
represents a stationary sequence after d-
order differentiation of time series 𝑋
, 𝛼
, 𝛼
,..., 𝛼
is the coefficient of the autoregressive part. 𝛽
, 𝛽
,...,
𝛽
is the coefficient of the moving average part. 𝜀
is
a random error term. p and q are the orders of
coefficients in formula (1). And d is the order of the
difference.
To determine this parameter, the common method
is the autocorrelation function (ACF) and partial
autocorrelation function (PACF). Figure 3 is the ACF
graph. The horizontal coordinate represents the lag
order, which is the time interval between the current
value and its lag value in the time series, while the
ordinate coordinate represents the autocorrelation
coefficient and is the intensity of the correlation
between the current value and the hysteresis value.
And Figure 4 is the PACF graph. The meaning of the
horizontal coordinate is the same as the ACF graph.
The vertical coordinate represents a PACF
coefficient, which is the pure correlation between the
current value and the hysteresis value after the
influence of the intermediate lag term is eliminated.
Figure 3: Autocorrelation graph (acf) (Photo/Picture credit:
Original).
Figure 4: Partial Autocorrelation graph (pacf)
(Photo/Picture credit: Original).
From the test analysis, the q value can be
determined from the ACF graph. It is at the first-order
truncated end, so the q is 1. The p-value can be
determined from the PACF graph. It is at the second-
order truncated end, so the p is 2. According to the
above analysis, d can be determined that d is 3. So,
the final selected model is ARIMA (1, 2, 3).
3.3 Residual Examination
After the modeling is completed, a white noise test is
also needed to ensure that the correlation in the price
change rate has been extracted. The residual is the
residual signal after the original signal is subtracted
from the signal fitted by the model. If the distribution
of residual is randomly normal and unautocorrelated,
it means that the residual is a white noise signal,
which also means that the useful signal has been
extracted into the ARIMA model.
The Ljung-Box test results of the predicted data
were obtained, showing that the p-value was 0.57
(much greater than 0.05), indicating that the residual
has no significant autocorrelation when the hysteresis
order 1. At the same time, the JB results show that the
p-value is 0.27 (also greater than 0.05), indicating that
the residual is approximately normal distribution and
meets the ARIMA model hypothesis. The test results
show that the residuals of the model are all white
noise, which means the model has extracted all the
information in the sequence.
3.4 Model Prediction
Finally, judging from the comparative trend line. The
UCL in Figure 5 represents 95% upper confidence
level, while the LCL represents a 95% lower
confidence level. Before 2024, the blue solid line
represents the fitting of past values, and the red
implementation represents the real value (almost
covered by the fitted value). After 2024, the blue solid