Sample, Skip and Switch – The Basics of Risk-based Incoming Inspection

Manostaxx - Industrial Management Consulting
Manostaxx – Industrial Management Consulting

You’re probably conducting incoming inspections today but if you aren’t taking a risk-based approach, chances are you’re performing inspections you don’t need to do and possibly overlooking the items which need to be inspected most.  Implementing a risk-based sampling system helps you spend less time and less money inspecting high-quality raw materials when you are already confident that they are good. You’re also able to pinpoint poorly performing raw materials and suppliers based on their inspection state. A typical risk-based inspection program includes a sampling system with a sampling plan, skip lot schedule, and inspection state switching rules. If you’re thinking about implementing a sampling system for incoming inspection, here are some basics to help you on your way.

The Sampling Plan and AQL – Manage Inspection Quantity

Sampling plans help to define your sample size or the number of items to be inspected for each lot of incoming material. When you choose a sampling plan, like the commonly used ANSI/ASQ Z1.4, you’re setting a baseline for the percentage of defects you’re willing to tolerate for each measured characteristic. This percentage may be 0 for your most critical product characteristics. For less critical characteristics, you may choose a different percentage (typically between 2.5-4.0%). This level is called your Acceptance Quality Level, or AQL. Based on your AQL, the sampling plan defines how many items to inspect for a given lot size. Typically, the lower the AQL, the higher the sample size you’ll need to have statistical confidence in a material’s quality. The sampling plan also defines how many defects can occur for the lot to still pass inspection.

The Skip Lot Schedule – Control Inspection Frequency

A skip lot schedule details how many lots need to be inspected out of the total you receive. Just like the sampling plan, your skip lot schedule can vary based on past supplier performance or based on a material’s risk. You may decide to vary the inspection schedule for certain raw materials, while you may have other materials that need no inspection at all.

Switching Rules – Performance-Based Change

Switching rules are a third component of risk-based incoming inspection. These rules take ongoing inspection results into consideration and tell you how your sample size and skip lot schedule should change as a result. For sampling systems like ANSI/ASQ Z1.4, switching rules govern changes between the following states:

  • Normal – The baseline for sample size and number of lots to be inspected
  • Reduced – Fewer lots are inspected and fewer samples are taken for each lot. This state is reached based on good results in previous incoming inspections.
  • Tightened – More lots are inspected and more samples are taken for each lot. This state is reached based on poor results in previous incoming inspections.

Switching rules are also based on a statistical plan that modifies the inspection schedule while maintaining confidence that incoming quality will remain high. For this reason, you will need to stick with an industry-standard plan rather than creating your own skip lot schedule.

The Challenge of Risk-Based Inspection

A major obstacle to risk-based inspection is that sampling systems are difficult to track. Industry standard sampling tables are detailed and complex. Plus, manual inspection management processes make it challenging to understand each material’s quality performance over time. This makes it difficult to determine sample size, skip lot frequency, and switching schedules for an organization’s many suppliers and raw materials.  Fortunately, electronic Inspection Management solutions can completely automate risk-based inspection processes. Tools like SmartSolve Supplier Quality Management combine industry-standard sampling systems with your ongoing supplier performance data to notify your team when inspection is needed. You can have all of the benefits of risk-based inspection without the administrative (or statistical) headaches.


SmartSolve® Supplier Quality Management

5-minute Overview Video

Take a risk-based approach to supplier quality. Watch this video to learn how SmartSolve Supplier Quality Management maximizes supplier quality while reducing your incoming inspection effort.

Sampling Systems and Compliance

In the Life Sciences, compliance has to be at the forefront of every incoming quality decision. So it is important to consider the compliance aspects of your sampling choices. It is critical that you clearly document the sampling system you decide to use, and that the sampling system you use is statistically valid. It is also important to remember that sampling systems should only be implemented if you have confidence in a supplier’s previous quality performance. So be sure that you’ve documented any audits, inspections, nonconformance history, or other records in support of this decision within your quality management system.

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