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What is it: The NP-Chart 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 Np-Chart 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 "P-Chart" and an "Np-Chart". A P-Chart is one that shows the fraction defective (p), whereas the NP-Chart shows the NUMBER of defectives (Np). They are practically the same thing with the exception that an NP-Chart is used when the size of the subgroup (N) is constant, and a P-Chart is used when it is NOT constant.

Where to use it: In industrial statistics, the NP-Chart is a type of control chart that is very similar to the P-Chart except that the statistic being plotted is a number count rather than a sample proportion of items. For example, an NP-Chart often shows the number of nonconforming items in each sample. Since we are counting failures or successes, clearly the appropriate data for NP-charts 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: NP-Chart overview:

  1. The "NP" stands for the number of nonconforming items, which can be expressed as n (sample size) times P (proportion of nonconforming items)
  2. Need a good definition of nonconforming items – usually a categorical definition
  3. Subgroup size must be constant
  4. Normally need large subgroups – can even be up to total for the period

Control limits for the NP-Chart are calculated on the basis of the binomial distribution and an approximation based on the central limit theorem.

Steps In Constructing a NP-Chart

  1. Collect the data recording the number inspected (N) and the number of defective products (NP). Divide the data into subgroups.
  2. Record the number of defectives on a chart or spreadsheet, along with the subgroup size.
  3. Record the number of defectives for each subgroup and record on the data sheet. Then total both columns.
  4. Compute the Control Limits.
  5. Draw in the Control Limits and plot the number of defective parts listed in our chart above. Connect the dots and observe the chart to determine if there are any points out of the control limits.

Preview (Click to enlarge)
  NP-Charting Tool Template
Microsoft Excel Format
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  NP-Chart Tool Graph
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  NP-Chart Production Template
Adobe PDF Format
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np Chart Production Template
USD $14.95
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