# Why do time series have to be stationary before analysis?

Time series analysis is an effective method for identifying and forecasting trends in data that have been gathered over time. Each data point in a time series represents a particular point in time, and the data is gathered over time. Time series data examples include stock price data, weather information, and website traffic. In a number of disciplines, including economics, finance, and weather forecasting, time series data is often employed.

The practice of utilizing statistical methods to comprehend the data over time and make predictions about it is known as time series analysis. The ability to spot patterns, trends and linkages in the data that can be utilized to forecast future outcomes makes time series analysis crucial.

When a time series is said to be stationary, it means that its statistical characteristics have remained stable over time. If the data's mean, variance, and autocorrelation structure do not vary over time, a time series is regarded as stationary. The statistical behavior of a stationary time series will be consistent across time, to put it another way. In this blog article, we'll look more closely at the need for time series to be stationary before analysis as well as what it actually implies.

## What is Stationarity in Time Series and how to test for stationarity in it?

The statistical characteristics of the data in a time series are said to be stationary if they stay stable throughout the course of time. A time series is regarded as stationary if its mean, variance, and autocorrelation structure do not alter over time. To put it another way, a stationary time series will have a steady statistical trend across time.

Among a stationary time series' characteristics are the following −

• A constant mean, i.e., a time series' average value that remains constant over the course of time.

• A constant variance indicates that the data's dispersion is consistent across time.

• A lack of trend or seasonality, which denotes that there is no upward or downward trend in the data and no predictable patterns that reoccur over a set time period.

The most popular techniques for determining stationarity in a time series are as follows −

• A statistical test that can be used to check for stationarity is the Augmented Dickey-Fuller test (ADF).

• A statistical test called the Kwatkowski-Phillips-Schmidt-Shin test (KPSS) can be used to check for stationarity.

• Examining the time series plot visually and breaking it down into its trend, seasonal, and residual components.

These tests, which are often employed in time series analysis, can be used to assess if a time series is stationary or not. It's crucial to remember that the unique problem and the domain expertise, in addition to the test findings, should be taken into account.

## But, why do time series have to be stationary before analysis?

The fact that many time series analysis techniques presuppose stationarity is one of the key reasons why time series must be stable before analysis. For instance, the widely used ARIMA (Auto-Regressive Integrated Moving Average) model for forecasting makes the assumption that the data is stationary. The model will not be able to faithfully reflect the underlying patterns in the data if it is non-stationary, and the findings will be erroneous.

The fact that non-stationarity might result in incorrect or misleading results is another justification for the necessity of time series being stationary before analysis. As an illustration, a non-stationary time series could give the impression that there is a high connection between two variables, but in reality, the correlation is only there because of a trend or a seasonal element in the data.

## How to make time series stationary?

A time series can be made stationary using a variety of methods, including −

• Differencing − This method includes taking the trend component out of the data by subtracting successive observations from one another.

• Seasonal Decomposition of Time Series (STL) − This approach divides a time series into trend, seasonal, and residual components.

• Log transformations − This method can be used to reduce the trend component and stabilize the variance.

It is important to keep in mind that keeping a time series stationary can also be a trade-off because the trend or seasonal component can cause some information to be lost. Making a series stationary should be done with caution since it might add biases and provide incorrect findings.

## Conclusion

To summarize, time series must be steady before analysis since many time series analysis techniques rely on stationarity and non-stationarity might result in incorrect or misleading findings. It is possible to make the time series stationary by using methods like differencing, STL, and log transformations. The unique problem, the quantity and caliber of the data provided, and the approach chosen all influence the results.