What is 'Autocorrelation'
Autocorrelation is a mathematical representation of the degree of similarity between a given time series and a lagged version of itself over successive time intervals. It is the same as calculating the correlation between two different time series, except that the same time series is used twice  once in its original form and once lagged one or more time periods.
The term can also be referred to as "lagged correlation" or "serial correlation".
BREAKING DOWN 'Autocorrelation'
When computed, the resulting number can range from +1 to 1. An autocorrelation of +1 represents perfect positive correlation (i.e. an increase seen in one time series will lead to a proportionate increase in the other time series), while a value of 1 represents perfect negative correlation (i.e. an increase seen in one time series results in a proportionate decrease in the other time series).
This value can be useful for computing for security analysis. For example, if you know a stock historically has a high positive autocorrelation value and you witnessed the stock making solid gains over the past several days, you might reasonably expect the movements over the upcoming several days (the leading time series) to match those of the lagging time series and to move upwards.

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