What is it: The NPChart is used for monitoring the number of times a condition occurs, relative to a constant sample size, when each sample can either have this condition, or not have this condition. The NpChart monitors the number of times a condition occurs, relative to a constant sample size, when each sample can either have this condition, or not have this condition. For our example, we would sample a set number of transactions each month from all the transactions that occurred, and from this sample count the number of transactions that had one or more errors. We would then track on the control chart the number of transactions with errors per month.
Why use it: The Np Control Chart is used to determine if the rate of nonconforming product is stable, and will detect when a deviation from stability has occurred. There is a difference between a "PChart" and an "NpChart". A PChart is one that shows the fraction defective (p), whereas the NPChart shows the NUMBER of defectives (Np). They are practically the same thing with the exception that an NPChart is used when the size of the subgroup (N) is constant, and a PChart is used when it is NOT constant. Where to use it: In industrial statistics, the NPChart is a type of control chart that is very similar to the PChart except that the statistic being plotted is a number count rather than a sample proportion of items. For example, an NPChart often shows the number of nonconforming items in each sample. Since we are counting failures or successes, clearly the appropriate data for NPcharts need to be attribute data. The subgroup size must be constant, as comparisons of counts would otherwise be meaningless. When to use it: To measure the number of defective items along a time sequence. How to use it: NPChart overview:
Control limits for the NPChart are calculated on the basis of the binomial distribution and an approximation based on the central limit theorem. Steps In Constructing a NPChart
