Home / Statistical Tools / Analysis Tools / Time Series / Autocorrelation
Autocorrelation¶
Measure how a series correlates with its own past values at each lag.
Where to find it
QXL Stat Tools → Analysis Tools → Time Series → Autocorrelation
Worked example
The Autocorrelation walkthrough builds this analysis step by step from sample data you can copy straight into Excel, and states every number it produces.
All six Time Series walkthroughs are listed on the How-To page.
What it does¶
The autocorrelation function (ACF) computes, for each lag \(k\), the correlation between the series and itself shifted \(k\) periods back. Spikes outside the confidence bands indicate meaningful time dependence; the pattern of spikes helps identify seasonality and choose forecasting models.
Options¶
The dialog is Time Series Correlation, with a Data tab and a Correlation Options tab.
Data tab
- Excel / GroupBy: analyze flat Excel columns, or split by one or more GroupBy columns
- Data Columns: one or more numeric columns; each produces its own ACF
Correlation Options tab
- Auto / Manual lag type: Auto lets Quantum XL pick how many lags to compute; Manual uses your lag count
- Lag Value: the number of lags when Manual is selected. Default 1; allowed 1 up to the number of observations minus 1.
Combining into a single chart is not available for the correlation tools.
Output¶
A worksheet named ACF ... containing:
- A needle-style chart of the ACF against Lag, with confidence bands
- A data table with Lag, ACF, T-Stat, LBQ columns (LBQ is the Ljung-Box Q statistic)