# Time Series Autocorrelation function (ACF)

This online calculator computes autocorrelation function for given time series and plots correlogram

**Autocorrelation**, also known as **serial correlation**, is the correlation of a signal with a delayed copy of itself as a function of delay. Informally, it is the similarity between observations as a function of the time lag between them. Autocorrelation analysis is a mathematical tool for finding repeating patterns, such as the presence of a periodic signal obscured by noise or identifying the missing fundamental frequency in a signal implied by its harmonic frequencies. It is often used in signal processing for analyzing functions or series of values, such as time-domain signals.

In statistics, the autocorrelation of a random process is the Pearson correlation between the process's values at different times, as a function of the two times or of the time lag. ^{1}

The sample Pearson correlation coefficient between *x* and *y* is:

For autocorrelation, this coefficient is computed between the time series and the same time series lagged by specified number of periods. For example, for 1-period time lag, the correlation coefficient is computed between first *N-1* values, i.e. and next *N-1* values (values shifted by one), i.e. .

,

where is the mean of the first *N-1* values, and is the mean of the last *N-1* values.

If we ignore difference between and , we can simplify the formula above to

This can be generalized for values separated by *k* periods as:

The value of is called the autocorrelation coefficient at lag . The plot of the sample autocorrelations versus (the time lags) is called the **correlogram** or **autocorrelation plot**.

The correlogram is a commonly used tool for checking randomness in a data set. Computing autocorrelations ascertain this randomness for data values at varying time lags. If random, such autocorrelations should be near zero for any and all time-lag separations. If non-random, then one or more of the autocorrelations will be significantly non-zero.

Also, correlograms are used in the model identification stage for Box–Jenkins autoregressive moving average time series models. Autocorrelations should be near-zero for randomness; if the analyst does not check for randomness, then the validity of many of the statistical conclusions becomes suspect. The correlogram is an excellent way of checking for such randomness.^{2}

The default data for the calculator below is obtained by noising sine function using Noisy function generator, and you can clearly see a non-random pattern.

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