A method of smoothing a time series to reduce the effects of random variation and reveal any underlying trend or seasonality. For the time series x1, x2,…, xt the simple three-point moving average would replace the value of xk, k=2, 3,…, t−1, with
Often, different weights are used, as in this five-point moving average which could be used for k=3, 4,…, t−2:
Another possibility is provided by Daniell weights: in the case of an average over m time points, the two end points are given weight , with the others each being given weight .
The four-point moving averages (appropriate for quarterly data) are
Twelve-point moving averages are similarly defined and are appropriate for monthly data.
For a cycle with an even period, e.g. quarterly or monthly data, the centred moving averages are the arithmetic means of the successive moving averages as defined above. For example, in the case of quarterly data the first centred moving average is
The advantage of these centred moving averages is that the resulting values are associated with a time point rather than the midpoint of the interval between two successive time points.
A graph of moving averages against time may show changes against time which are obscured by cyclical effects. A line of best fit to the moving averages is a trend line, and its slope is the trend. The trend line may be used to forecast future values (in the short term). For example, for monthly data the average deviation of the January data from the trend line can be used as an estimate of the future deviation of the January deviation from the trend line. The deviation can be measured as either a difference or a ratio.
Note that the use of moving averages can introduce spurious cycles (see Slutsky–Yule effect).