SPC Control Charts

Variable Control Charts

Variable Control charts are for use with sampled quality data that can be assigned a specific numeric value, other than just 0 or 1. This might include, but is not limited to, the measurement of a critical dimension (height, length, width, radius, etc.), the weight a specific component, or the measurement of an important voltage. Common types of Variable Control Charts include X-Bar R (Mean and Range), X-Bar Sigma, I-R (Individual Range), EWMA, MA, MAMR (Moving Average/Moving Range), MAMS (Moving Average/Moving Sigma), and Levey-Jennings charts. You will find information about the different Variable Control charts in the links below:

Attribute Control Charts

Attribute Control Charts are a set of control charts specifically designed for tracking defects (also called non-conformities). These types of defects are binary in nature (yes/no), where a part has one or more defects, or it doesn’t. Examples of defects are paint scratches, discolorations, breaks in the weave of a textile, dents, cuts, etc. Think of the last car that you bought. The defects in each sample group are counted and run through some statistical calculations. Depending on the type of Attribute Control Chart, the number of defective parts are tracked (p-chart and np-chart), or alternatively, the number of defects are tracked (u-chart, c-chart). The difference in terminology “number of defective parts” and “number of defects” is highly significant, since a single part not only can have multiple defect categories (scratch, color, dent, etc), it can also have multiple defects per category. A single part may have 0 – N defects. So keeping track of the number of defective parts is statistically different from keeping track of the number of defects. This affects the way the control limits for each chart are calculated. You will find information about the different Variable Control charts in the links below:

  • p-Charts – Percent (or Fraction) Defective Parts Chart (Fixed and Variable Sample Subgroup Size)
  • np-Chart – Number Defective Parts Chart
  • c-Chart – Number of Defects Chart
  • u-Chart – Number of Defects per Unit Chart – (Fixed and Variable Sample Subgroup Size)
  • DPMO-Chart – Defects per Million Opportunities Chart

Other Quality Control Charts

Quality engineers use other, specialized, charts in the analysis of SPC data.

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