# Arima Model in Python

ARIMA (AutoRegressive Integrated Moving Average) is a popular time series forecasting method that combines autoregression (AR), differencing (I), and moving average (MA) models. In Python, you can use the `statsmodels` library to build an ARIMA model. Here’s an example:

First, install the `statsmodels` library by running `pip install statsmodels` in your terminal.

Then, let’s load some example time series data from the `statsmodels` library:

```from statsmodels.datasets import sunspots
import matplotlib.pyplot as plt

plt.plot(data)
plt.title("Sunspot Data")
plt.show()
```

This will plot the sunspot data:

Now, we can fit an ARIMA model to this data:

```from statsmodels.tsa.arima.model import ARIMA

model = ARIMA(data, order=(5, 1, 0))  # p=5, d=1, q=0
results = model.fit()
print(results.summary())
```

Here, we’ve specified an ARIMA model with p=5 (AR order), d=1 (differencing order), and q=0 (MA order). The `fit()` method will fit the model to the data and return a summary of the model.

You can also use the `forecast()` method to make predictions:

```forecast = results.forecast(steps=12)
print(forecast)
```

This will make 12 future predictions based on the model.

That’s the basic idea of fitting an ARIMA model in Python using `statsmodels`. Of course, you’ll need to adjust the model order (p, d, q) and other parameters based on your specific time series data and problem.

### Based on the frequency, a Time Series can be classified into the following categories:

Based on the frequency of observations, a time series can be classified into the following categories:

1. Continuous Time Series: This type of time series is measured over a continuous time interval, such as temperature readings every second, stock prices every minute, or audio signals recorded in real-time.
2. Discrete Time Series: This type of time series is measured at discrete points in time, such as daily sales figures, monthly unemployment rates, or yearly GDP growth rates.
3. Regularly Spaced Time Series: In this type of time series, the observations are recorded at equally spaced time intervals, such as hourly, daily, weekly, or monthly intervals.
4. Irregularly Spaced Time Series: In this type of time series, the observations are recorded at irregular time intervals, such as stock prices recorded at random times during the day, or earthquake occurrences recorded whenever they happen.
5. Longitudinal or Panel Time Series: This type of time series involves multiple observations of the same variable(s) over time for multiple individuals, groups, or entities. For example, a panel dataset of the stock prices of multiple companies over time.
6. Event Time Series: This type of time series is characterized by the occurrence of events at irregular intervals, such as natural disasters, political crises, or social unrest.

Understanding the frequency of a time series is important because it can impact the type of analysis that is appropriate, the forecasting methods that can be used, and the interpretation of the results.

### However, what is the need for forecasting?:

Forecasting is a technique used to predict future values of a variable based on historical data and trends. The need for forecasting arises in many different situations, including:

1. Business Planning: Forecasting helps businesses to plan for the future by predicting demand for their products or services, and making decisions about production, staffing, inventory, and pricing.
2. Budgeting and Financial Planning: Forecasting helps organizations to predict future revenues, expenses, and cash flows, and make informed decisions about budgeting and financial planning.
3. Supply Chain Management: Forecasting helps companies to manage their supply chains by predicting demand for raw materials, forecasting inventory levels, and planning production schedules.
4. Risk Management: Forecasting helps organizations to manage risk by predicting potential outcomes, identifying potential problems, and developing contingency plans.
5. Marketing and Sales: Forecasting helps companies to predict sales volumes, identify trends and opportunities, and make informed decisions about marketing and sales strategies.
6. Public Policy and Planning: Forecasting helps governments and public organizations to plan for the future by predicting population growth, demand for services, and economic trends.

Overall, forecasting is an important tool for organizations and individuals who need to make informed decisions about the future. By using historical data and trends to predict future outcomes, forecasting can help to reduce uncertainty, manage risk, and make more informed decisions.

### An Introduction to ARIMA Models:

ARIMA (Autoregressive Integrated Moving Average) models are a popular class of statistical models used for time series analysis and forecasting. ARIMA models are used to capture the temporal structure of a time series by incorporating three components: autoregression, differencing, and moving averages.

The ARIMA model can be denoted as ARIMA(p, d, q), where:

• p is the order of autoregression (AR), which represents the number of lagged values of the time series that are included in the model.
• d is the order of differencing (I), which represents the number of times the time series is differenced to make it stationary (i.e., removing trends and seasonality).
• q is the order of moving average (MA), which represents the number of lagged forecast errors that are included in the model.

The AR component of an ARIMA model represents the dependence of the current value of the time series on its past values, while the MA component represents the dependence of the current value of the time series on the past forecast errors. The I component is used to make the time series stationary, which is necessary for applying the AR and MA components.

ARIMA models are widely used in fields such as economics, finance, engineering, and meteorology for analyzing and forecasting time series data. These models can be used to make short-term and long-term forecasts, and can also be used to identify trends, seasonal patterns, and irregularities in the data.

There are various methods and techniques available to estimate the parameters of an ARIMA model, including maximum likelihood estimation, least squares estimation, and Bayesian estimation. The appropriate method depends on the specific characteristics of the data and the objective of the analysis.

Overall, ARIMA models provide a powerful tool for time series analysis and forecasting, and are widely used in practice.

### Understanding Auto-Regressive (AR) and Moving Average (MA) Models:

Auto-Regressive (AR) and Moving Average (MA) models are two important components of ARIMA models, and are used to capture the temporal dependence of a time series.

Auto-Regressive (AR) Models: An Auto-Regressive (AR) model uses past values of a time series to predict future values. In an AR model of order p, the current value of the time series is regressed on its p past values (i.e., the lagged values of the time series). The order of the AR model, denoted as AR(p), represents the number of lagged values included in the model.

The AR model can be expressed mathematically as:

Y_t = c + ϕ1 * Y_t-1 + ϕ2 * Y_t-2 + … + ϕp * Y_t-p + ε_t

where Y_t is the current value of the time series, ϕ1, ϕ2, … , ϕp are the coefficients of the lagged values, c is a constant, ε_t is the error term, and p is the order of the AR model.

The AR model assumes that the future values of the time series are dependent on its past values, and that the magnitude of the dependence decreases as the lag increases.

Moving Average (MA) Models: A Moving Average (MA) model uses past forecast errors of a time series to predict future values. In an MA model of order q, the current value of the time series is regressed on its q past forecast errors (i.e., the difference between the actual value and the predicted value of the time series). The order of the MA model, denoted as MA(q), represents the number of forecast errors included in the model.

The MA model can be expressed mathematically as:

Y_t = c + ε_t + θ1 * ε_t-1 + θ2 * ε_t-2 + … + θq * ε_t-q

where Y_t is the current value of the time series, ε_t is the error term, θ1, θ2, … , θq are the coefficients of the past forecast errors, c is a constant, and q is the order of the MA model.

The MA model assumes that the future values of the time series are dependent on its past forecast errors, and that the magnitude of the dependence decreases as the lag increases.

AR and MA models can be combined to create ARMA models, which incorporate both past values of the time series and past forecast errors to predict future values. ARIMA models are an extension of ARMA models that include differencing to make the time series stationary.

### Finding the order of differencing ‘d’ in the ARIMA Model:

Finding the appropriate order of differencing (d) in the ARIMA model is important for achieving a stationary time series, which is a key assumption of the ARIMA model. Here are some common methods for determining the order of differencing:

1. Augmented Dickey-Fuller (ADF) Test: The ADF test is a statistical test used to determine whether a time series is stationary or not. The test outputs a test statistic and a p-value. If the p-value is less than a chosen significance level (e.g., 0.05), then the null hypothesis of non-stationarity is rejected, indicating that the time series is stationary. If the p-value is greater than the significance level, then the null hypothesis cannot be rejected, indicating that the time series is non-stationary.

One common approach for determining the order of differencing is to apply the ADF test to the time series with increasing values of d until a stationary time series is obtained. The minimum value of d required to achieve stationarity can be considered as the appropriate order of differencing.

2. Visual Inspection: Another approach for determining the order of differencing is to plot the time series and visually inspect for trends, seasonality, and irregularities. If the plot indicates the presence of a trend or seasonality, then differencing can be applied until these features are removed and a stationary time series is obtained. However, this approach can be subjective and less reliable compared to statistical tests.

3. Partial Autocorrelation Function (PACF): The PACF is a statistical tool used to identify the order of autoregression (p) in the ARIMA model. However, the PACF can also be used to determine the order of differencing. If the PACF shows a significant spike at lag 1, then the time series may require one order of differencing. If the PACF shows a significant spike at lag 2, then the time series may require two orders of differencing, and so on.

It is important to note that the appropriate order of differencing may vary depending on the specific characteristics of the time series and the objective of the analysis. Therefore, it is recommended to try multiple methods and compare the results to determine the most appropriate order of differencing for the ARIMA model.

### Finding the order of the Auto-Regressive (AR) term (p):

Finding the appropriate order of the Auto-Regressive (AR) term (p) in the ARIMA model is important for capturing the temporal dependence of the time series. Here are some common methods for determining the order of the AR term:

1. Partial Autocorrelation Function (PACF): The PACF is a statistical tool used to identify the order of autoregression (p) in the ARIMA model. The PACF shows the correlation between the current value of the time series and its lagged values after removing the effects of the intermediate lags. If the PACF shows a significant spike at lag 1, then an AR model of order 1 (AR(1)) may be appropriate. If the PACF shows a significant spike at lag 2, then an AR model of order 2 (AR(2)) may be appropriate, and so on.

2. Akaike Information Criterion (AIC): The AIC is a statistical measure that balances the goodness of fit of a model with its complexity. It is calculated based on the likelihood function of the model and penalizes for additional parameters. A lower AIC value indicates a better balance between goodness of fit and complexity. One common approach for determining the order of the AR term is to fit multiple AR models with different values of p and choose the one with the lowest AIC value.

3. Bayesian Information Criterion (BIC): The BIC is similar to the AIC, but it imposes a stronger penalty on additional parameters, which leads to a more parsimonious model. One common approach for determining the order of the AR term is to fit multiple AR models with different values of p and choose the one with the lowest BIC value.

It is important to note that the appropriate order of the AR term may vary depending on the specific characteristics of the time series and the objective of the analysis. Therefore, it is recommended to try multiple methods and compare the results to determine the most appropriate order of the AR term for the ARIMA model.

### Finding the Order of the Moving Average (MA) term (q):

Finding the appropriate order of the Moving Average (MA) term (q) in the ARIMA model is important for capturing the impact of past errors on the current value of the time series. Here are some common methods for determining the order of the MA term:

1. Autocorrelation Function (ACF): The ACF is a statistical tool used to identify the order of the Moving Average (MA) term in the ARIMA model. The ACF shows the correlation between the current value of the time series and its lagged values. If the ACF shows a significant spike at lag 1, then an MA model of order 1 (MA(1)) may be appropriate. If the ACF shows a significant spike at lag 2, then an MA model of order 2 (MA(2)) may be appropriate, and so on.

2. Akaike Information Criterion (AIC): The AIC is a statistical measure that balances the goodness of fit of a model with its complexity. It is calculated based on the likelihood function of the model and penalizes for additional parameters. A lower AIC value indicates a better balance between goodness of fit and complexity. One common approach for determining the order of the MA term is to fit multiple MA models with different values of q and choose the one with the lowest AIC value.

3. Bayesian Information Criterion (BIC): The BIC is similar to the AIC, but it imposes a stronger penalty on additional parameters, which leads to a more parsimonious model. One common approach for determining the order of the MA term is to fit multiple MA models with different values of q and choose the one with the lowest BIC value.

It is important to note that the appropriate order of the MA term may vary depending on the specific characteristics of the time series and the objective of the analysis. Therefore, it is recommended to try multiple methods and compare the results to determine the most appropriate order of the MA term for the ARIMA model.

### Handling the A Slightly Under or Over-Differenced Time Series:

If a time series is slightly under or over-differenced, meaning that it is not perfectly stationary but has already been differenced, there are a few possible techniques that can be used to handle it:

1. Add an Integrated (I) Term: If the time series is slightly under-differenced, meaning that there is still some trend or seasonality present, then adding an integrated (I) term to the ARIMA model may be helpful. This means that the time series will be differenced one more time to make it stationary. However, adding too many differences can result in over-differencing, so it is important to carefully assess the need for an additional difference.
2. Adjust the Order of the AR and MA Terms: If the time series is slightly over-differenced, meaning that it is too stationary, then adjusting the order of the AR and MA terms in the ARIMA model may be helpful. For example, if the time series is over-differenced with a high degree of differencing (d), then decreasing the order of the differencing term and increasing the order of the autoregressive (AR) or moving average (MA) terms can help capture the remaining non-stationary components.
3. Try Other Models: If adjusting the order of the AR and MA terms does not help, then it may be helpful to try other time series models, such as exponential smoothing, which do not require differencing. Another option is to explore data transformations, such as taking the logarithm or square root of the time series, to stabilize the variance.

Overall, it is important to carefully assess the nature of the time series and the need for additional differencing or adjustments to the ARIMA model. In some cases, other time series models may be more appropriate for the specific characteristics of the time series.

### Building the ARIMA Model:

To build an ARIMA model for time series forecasting, follow these steps:

1. Visualize the Data: Start by visualizing the time series data to understand its patterns and characteristics. Look for trends, seasonality, and any unusual fluctuations or outliers.
2. Test for Stationarity: Check for stationarity of the time series using statistical tests such as the Augmented Dickey-Fuller (ADF) test. If the time series is not stationary, then take the appropriate steps to make it stationary, such as differencing.
3. Determine the Order of Differencing (d): If differencing is needed, determine the order of differencing (d) using methods such as the Augmented Dickey-Fuller (ADF) test or the visual inspection of the autocorrelation function (ACF) and partial autocorrelation function (PACF).
4. Determine the Order of the Autoregressive (AR) Term (p): Determine the order of the autoregressive (AR) term (p) using methods such as the visual inspection of the partial autocorrelation function (PACF) or the Akaike Information Criterion (AIC).
5. Determine the Order of the Moving Average (MA) Term (q): Determine the order of the moving average (MA) term (q) using methods such as the visual inspection of the autocorrelation function (ACF) or the Akaike Information Criterion (AIC).
6. Fit the ARIMA Model: Using the determined values of d, p, and q, fit the ARIMA model to the time series data. This involves estimating the model parameters using techniques such as maximum likelihood estimation.
7. Evaluate the Model: Evaluate the performance of the ARIMA model using metrics such as the Mean Absolute Error (MAE), Mean Squared Error (MSE), and Root Mean Squared Error (RMSE). Visualize the predicted values against the actual values to assess the accuracy of the model.
8. Make Forecasts: Once the ARIMA model is deemed accurate and appropriate, use it to make forecasts of future values of the time series.

Overall, building an ARIMA model requires careful analysis of the time series data and the selection of appropriate values for the order of differencing, autoregressive term, and moving average term. Once the model is built and evaluated, it can be used to make forecasts and inform decision-making.