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**Autocorrelation** measures the degree of similarity between a time series and a lagged version of itself over successive time intervals.

It’s also sometimes referred to as “serial correlation” or “lagged correlation” since it measures the relationship between a variable’s current values and its historical values.

When the autocorrelation in a time series is high, it becomes easy to predict future values by simply referring to past values.

**How to Calculate Autocorrelation in R**

Suppose we have the following time series in R that shows the value of a certain variable during 15 different time periods:

#define data x

We can calculate the autocorrelation for every lag in the time series by using the **acf()** function from the **tseries **library:

library(tseries) #calculate autocorrelations acf(x, pl=FALSE) 0 1 2 3 4 5 6 7 8 9 10 1.000 0.832 0.656 0.491 0.279 0.031 -0.165 -0.304 -0.401 -0.458 -0.450 11 -0.369

The way to interpret the output is as follows:

- The autocorrelation at lag 0 is
**1**. - The autocorrelation at lag 1 is
**0.832**. - The autocorrelation at lag 2 is
**0.656**. - The autocorrelation at lag 3 is
**0.491**.

And so on.

We can also specify the number of lags to display with the **lag **argument:

#calculate autocorrelations up to lag=5 acf(x, lag=5, pl=FALSE) Autocorrelations of series 'x', by lag 0 1 2 3 4 5 1.000 0.832 0.656 0.491 0.279 0.031

**How to Plot the Autocorrelation Function in R**

We can plot the autocorrelation function for a time series in R by simply not using the **pl=FALSE** argument:

#plot autocorrelation function acf(x)

The x-axis displays the number of lags and the y-axis displays the autocorrelation at that number of lags. By default, the plot starts at lag = 0 and the autocorrelation will always be **1 **at lag = 0.

You can also specify a different title for the plot by using the **main **argument:

#plot autocorrelation function with custom title acf(x, main='Autocorrelation by Lag')

**Additional Resources**

How to Calculate Autocorrelation in Python

How to Calculate Autocorrelation in Excel