Exponentially Weighted Moving Average (EWMA) Chart

Instead of plotting the simple averages of a particular numbers of successive sample means, these means are weighted so that historically "older" means are assigned increasingly smaller weights. This is useful for detecting small permanent shifts (trends) in the process averages.

In the illustration above, the EWMA chart is shown with an R Chart and histograms of the EWMA and Ranges. The STATISTICA Quality Control module also provides options for creating S charts. Additionally, EWMA charts can be constructed for individual observations (N=1), in which case sigma will be estimated from moving ranges.

For information on how EWMA charts and their control limits are calculated, see QCC Computational Details. See also, Common Types of Charts and Control Chart for Individual Observations.