Capability Rollup Report

Summary

A tool for assessing the combined capability of any system. The Capability Rollup Report can combine (roll up) capabilities from process steps or product components, which can have either continuous or discrete data. The report contains a detailed rollup analysis showing the capability of the individual components and combined capability measures (for example, throughput yield, rolled throughput yield, and DPMO) for the individual steps and components as well as the combined system.

Answers the questions:
  • What is the combined (rolled up) capability of several process steps, subprocesses, subassemblies, and so on?
  • Which process steps, subprocesses, or subassemblies have the greatest need for improvement?
  • What is the overall capability of the process (both long-term and short-term) at the start of the process improvement project?
  • What is the overall capability of the process (both long-term and short-term) after improvements have been made?
  • What is the capability for a single process step with discrete data?
  • What is the probability of performing a step or producing a defect-free component the first time?
  • What is the probability of performing all steps or producing all defect-free components for a product or service the first time?
When to Use Purpose
Pre-project Assist in project selection by focusing on portions of a process or a product that have high defect rates.
Start of project Perform a baseline capability analysis (for processes with discrete data) on the process to determine the capability of the overall process and its components at the start of the project. The baseline analysis also helps set improvement goals for the project.
Mid-project Perform a confirmation capability analysis (for processes with discrete data) after improvements have been implemented to confirm that the process performs as predicted.
End of project Perform a capability analysis (for processes with discrete data) after implementing controls to obtain a final assessment of process capability, and also to determine whether the project attained its improvement goals.

Data

Summarized data: defects, opportunities, units, DPMO, and Z shift.

How-To

  1. The report has two tables: one for discrete outputs and the other for continuous outputs.
  2. Each step or component you will combine in the Capability Rollup Report either results in continuous or discrete outputs.
  3. Enter the name of each step or component into the appropriate table.
  4. Optionally, enter the number of times you must execute the step, or produce the component, to produce one unit of finished product or service. The default value is 1.
  5. In the Discrete Outputs table, you must enter (for each step or component) the number of units tested, the number of defects observed, and the number of opportunities for defects per unit.
  6. In the Continuous Outputs table, you must enter (for each step or component) the DPMO (long-term), which is usually obtained from a capability analysis, such as the Capability Analysis (Normal) analysis capture tool in Quality Companion.
  7. For both discrete and continuous outputs, you may also enter a value of Z shift, which is the difference between Zbench (ST) and Zbench (LT). This value is optional; however, unless it is provided, you will not obtain a Zbench (ST). If you do not know the value of Z shift, which is often the case, many Six Sigma deployments use a value of 1.5, which is the default used in both tables.

Guidelines

  • The Capability Rollup Report requires accurate counts of defects, units, and the number of opportunities per unit for any step or component with discrete data. The opportunities per unit are often the most difficult to determine; the lower the opportunity count, the more critical it is to be accurate.
  • At times, the number of units that pass through certain process steps is not in the usual proportions, due to various business factors. For example, if you shut down a portion of your operation for maintenance, you might speed up the process prior to the shutdown. Disproportionate unit counts can produce inaccurate results for the overall performance measures. The Capability Rollup Report corrects these disproportions by balancing the number of units, defects, and opportunities based on the number of operations. The default number of operations for each step or component is 1. For process steps, each unit must pass through each step in the process one time. For components, a completed unit must have one of each component. If a unit must pass through a process step more than one time, or if a unit must have more than one component, change the number of operations accordingly.
  • The difference between the Capability Rollup Report and the capability reports in Minitab — Capability Sixpack (normal or nonnormal), Capability Analysis (Binomial), and Capability Analysis (Poisson) — is that the Capability Rollup Report combines data (either continuous or discrete) for multiple process steps into a system capability analysis, while the Minitab reports calculate capability for a single process step over time.
  • The first section of the Capability Rollup Report calculates the yield for each process step or component:
    • Step Ytp calculates the throughput yield for each process step or component. Throughput yield is the probability of performing the step or building the component without defects the first time (no scrap or rework). For continuous outputs, the number of opportunities per unit is always 1, which means that the YTP is 1 – DPMO.
    • Step Yrt calculates the rolled throughput yield for each process step or component. Rolled throughput yield is the probability of performing the step or building the component without defects the first time (no scrap or rework) based on the value you enter in # of Operations. Step Yrt is the Step Ytp raised to the power of the number of operations.
  • The last section of the report displays combined performance metrics:
    • Total DPMO is the opportunity-level defect rate for the system. To measure the overall capability of the system output, Yrt is the preferred metric because it takes the capability of all of the requirements in the system and combines them into a single measure. The total DPMO measures the capability of a typical system requirement without assuming that the process can produce the requirement at its target value and assuming the process is subject to special causes of variation.
    • Yrt is the rolled throughput yield for the entire system. It is the probability of completing all of the steps or producing all of the components necessary for one complete unit of finished product or service without defects the first time (no scrap or rework).
    • Defective Rate is the percentage of units that will pass through a set of steps (or receive a set of components) and will have one or more defects.
    • Z Bench (ST) is a measure of the capability of a typical system requirement. Like Total DPMO, Z Bench (ST) is not a measure of the overall capability of the system output. To measure the overall capability of the system output, Yrt is the preferred metric because it combines the capability of all of the requirements in the system into a single measure. Z Bench (ST) measures the capability of a typical system requirement, assuming the process produces the requirement at its target value, with no special causes of variation, and uses the current control mechanisms.
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