NP-ChartAttribute (Discrete) Control ChartsNP-Chart is an Attribute control chart used when plotting: 1) DEFECTIVES 2) BINOMIAL ASSUMPTIONS SATISFIED 3) CONSTANT (fixed) SAMPLE SIZE (subgroup size) Each observation is independent. Use this chart to develop upper and lower control limits (UCL and LCL) and determine performance of process over time. Plots the number of non-conforming units or pieces. Each piece or unit may have one or more defects that make it defective.
The fixed subgroup size allows for constant control limit values, the is not a need to recalculate the control limits for each sample size. If the defective level is very low, (sample size * average % defective < 5), then apply binomial distribution exact limits. Used to detect shifts >1.5 standard deviations. In order to detect smaller shifts there are other charts that can be applied to variable and attribute data such as Exponentially Weighted Moving Average (EWMA) and Cumulative Sum of Quality Characteristic Measurement (CUSUM). Below is sample set of data and the NP-Chart follows.
The chart indicates that the process is in control. The control tests that were used all passed in this case. Recall there are a variety of control test and most statistical software programs allow you to select and modify these criteria.
Reminders for using NP-Charts: Another Example to calculate NP-Chart Control LimitsWith 50 samples (n=50), a process has a p-bar of 0.25. What are the upper and lower three sigma limits (UCL/LCL)?A) 0.25 and -0.25 B) 12.5 and -12.5 C) 21.69 and 3.31 D) 15.56 and 9.44
Return to the MEASURE phase Return to the CONTROL phase Shop for SPC Charting material and literature Return to Six-Sigma-Material Home Page from NP-Chart
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