Quality Control Introductory Overview - Control Chart for Individual Observations

Variable control charts can be constructed for individual observations taken from the production line, rather than samples of observations. This is sometimes necessary when testing samples of multiple observations would be too expensive, inconvenient, or impossible. For example, the number of customer complaints or product returns may only be available on a monthly basis; yet, one would like to chart those numbers to detect quality problems. Another common application of these charts occurs in cases when automated testing devices inspect every single unit that is produced. In that case, one is often primarily interested in detecting small shifts in the product quality (for example, gradual deterioration of quality due to machine wear).

With STATISTICA, you can construct X, moving average (MA), and exponentially weighted moving average (EWMA) charts for individual observations; charts for cumulative sums (CuSum) of deviations of individual measurements from the centerline can also be constructed. In all of these charts, by default sigma will be estimated from moving ranges (MR) of consecutive observations, which are plotted in an MR chart. See Computational Details for additional information.