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| How are control limits calculated for an attribute np-chart? |
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An np-chart displays the number of nonconforming (defective) units. This chart is available in WinSPC to an attribute that is configured with an 'Attribute Counts' of 'Defective Units' and an 'Inspection Lot Size' of 'Fixed' (an np-chart requires that the lot size be constant).
An attribute displaying an np-chart in WinSPC (configured to recalculate the control limits every 5 subgroups) can be seen here:
The statistic plotted on an np-chart is
where  is the number of nonconforming units in the k-th subgroup.
The following equations are used to calculate the control limits for the k-th subgroup:
Where:
 is either
- the average of a set of plotted np values (taking into account the 'All Data', 'Last N subgroups', and 'Use local data only' options on the 'Control Limits' tab of the attribute properties), if the 'Est. based on process variables' option is selected under the 'Chart Statistic' area.
- or, the value entered for the 'Constant' option, if it is selected under the 'Chart Statistic' area.
 is the 'Number of sigma' option under the 'Control Limit Spread' area on the 'Control Limits' tab of the attribute properties. By default, this is 3.0.
 is the average percent of nonconforming units (in other words,  , where  is the constant lot size).
If the calculation for LCL results in a value less than zero, the LCL is set to zero.
Note that the control limits will recalculate at the interval defined by the 'Calculated every k subgroups' option (25 by default).
For reference for the above control limit settings, here is a sample 'Control Limits' tab of the attribute properties:
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