To ensure that your results are valid, consider the following guidelines when you collect data, perform the analysis, and interpret your results.
If you can determine only whether each item is defective or nondefective, use P Chart or Laney P' Chart to plot the proportion of defective items, or use NP Chart to plot the number of defective items.
Because control charts detect changes over time, the order of the data is important. You should enter the data in the order it was collected, with the oldest data at the top of the worksheet.
Collect data at equally spaced time intervals, such as every hour, every shift, or every day. Select a time interval that is short enough that you can identify changes to the process soon after the changes occur.
A subgroup is a collection of similar items that are representative of the output from the process you want to evaluate. A subgroup can be either a single unit or a collection of similarly sized units. For example, you could record the number of surface imperfections for one LCD panel (a single unit) or for a collection of LCD panels that are all the same size.
If the subgroup is a collection of units, they should be collected under the same process conditions, such as personnel, equipment, suppliers, or environment. If you don't collect data in subgroups where items are subject to the same process conditions, you may not be able to distinguish between common-cause and special-cause variation.
For example, an LCD manufacturer wants to monitor defects on 17-inch LCD screens. Technicians record the number of dead pixels for each subgroup of 10 screens per hour and use a C chart to monitor the number of dead pixels.
If the subgroups are not large enough, the control limits may not be accurate when they are estimated from the data. The required subgroup size () depends on the average number of defects per unit (). Use the following formula to determine whether your subgroups are large enough, . For example, if the average number of defects per unit is 0.06, then all subgroups must have at least 9 units: , rounded up to the nearest whole number = 9.
If you don't have enough subgroups, you can still use the control chart, but you should consider the results preliminary because the control limits may not be precise. If you are using the chart on an ongoing basis, re-estimate the control limits after you have collected enough subgroups.