Control limits are computed from the data: the centre line is the process average and the limits are three sigma on either side, estimated from the within-subgroup spread. Specification limits come from the customer and are drawn in their own style on the primary chart only; they are not used for the control limits or the run rules. All statistics are computed by Smart.Industrial.spc, so a capability figure in a report and the one under the chart are always the same.
The limits are established from the first fifteen subgroups and then held. Around subgroup eighteen the process shifts by about half a sigma, which is not enough to put a point outside the limits. Press Add subgroup to continue sampling the shifted process and see the run rules detect the shift before any point is out of control.
The points that violate a rule and the rule names are listed as text under the chart. The same list is available to screen readers.
When a process is measured one unit at a time there are no subgroups, so sigma is estimated from the moving range of successive readings. The moving range chart has one point fewer than the individuals chart.
A shift that inspects 80 units gives less information than one that inspects 400, so its control limits are wider. The component computes the limits for each sample size and draws them as steps.