The Western Electric (WECO) rules spc, are decision rules used in Statistical Process Control to detect “out-of-control” or non-random conditions. The location of the observations relative to the control chart limits (typically at +-3 standard deviations) and the centerline indicate whether the process should be investigated for assignable reasons.

These rules are based on multiple criteria and attempt to differentiate unnatural patterns from natural ones.

  • A mixture pattern is a lack of points close to the centerline.
  • A stratification pattern is a lack of points close to the control limits.
  • An instability pattern is a condition in which points are not within the control limits.
  • Other unnatural patterns (auto-correlative, repetition, trend patterns)

This is the last sustaining phase in the continuous improvement cycle. It’s best for an organization to have a visual control system or built-in engineering controls that can be used to monitor and verify that the process is being changed. Statistical Process Control allows for process analysis. Analysts can use simple rules to determine if the process they are focusing on is within acceptable limits.

Customers can request services from an organization if they feel that the company is capable and in control. These are quality tools that can be used to validate and facilitate customer requests. These rules can be used to identify out of control conditions and take corrective action when control charts are being analyzed.

The Western Electric Rules spc:

Process operations are subject to natural variation. Process analysts should be aware of data points that move from their expected centerline. This is when data analysis shows unnatural patterns. There are four types of unnatural patterns. These include mixture, stratification and instability. All-cause shifts are those that alter the data distribution. This can either shift the center line accumulation or move the data towards extreme control limits or outside the lower or upper control bands.

To interpret the data, statistical zone rules are used to identify the zones. These rules are:

Rule #1: This rule can be used if one data point is not in zone A.

Rule #2: In a sequence of values, on the same side as the centerline, if two or more of the three values fall within zone A or exceed control limits, rule number two is used.

Rule #3: In a sequence of five points on the same side as the centerline, if four of them are in zone A or B, then rule 3 applies.

Rule #4: If all points fall within zone C or higher, place rule 4.

Analyzing control charts can help an analyst determine which control zone and which control rule should be applied to their situation. The appropriate action can then be taken once the criteria have been identified.

Statistical process control can be applied to many fields, including the chemical and semiconductor industries and the medical industry. This quality management tool allows for the monitoring of many critical variables in each industry. Western Electric Rules is a powerful visual control tool. Make sure they are correctly implemented and used to create a system that is in control.