Six Sigma Product Report

Summary

Assesses combined capability of a process or product. This report is available only when the Six Sigma add-on module for Minitab is installed.

By recording the number of defects observed, the number of units that passed through the system, and the number of opportunities for defects at the unit level, you can calculate a detailed roll-up analysis. This analysis shows the capability of the individual components and combined capability measures (for example, throughput yield, rolled throughput yield, DPMO, and Z), graphs showing the relative capability and control of each component, and the contributions of complexity, capability, and control to the overall capability of the 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?
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 on the process to determine the capability of the overall process and its components at the start of the project. A baseline analysis helps you set improvement goals for the project.
Mid-project Perform a confirmation capability analysis after improvements have been implemented to confirm that the process performs as predicted.
End of project Perform a capability analysis after implementing controls to obtain a final assessment of process capability, and also to determine whether the improvement goals of the project were attained.

Data

Summarized data: defects, opportunities, units.

How-To

  1. Determine what constitutes an opportunity for defect, at the unit level, for each component in the system.
  2. In Minitab, enter the names of the components (process steps, parts, subassemblies, and so on) in one column of the worksheet.
  3. Enter the number of defects for each component, the number of units for each component, and the number of opportunities for defect per unit, for each component in three separate columns.
  4. (Optional) Enter a value for Z-shift (the difference between short-term and long-term capability) into a column. If you do not know the exact value of Z-shift, use 1.5 (a common value).
  5. (Optional) Enter the number of times the step must be executed, or the component produced, in order to produce one unit of finished product or service. Enter these values into a column, which Minitab calls Complexity.

Guidelines

  • You must have accurate counts of defects, units, and the number of opportunities per unit. The opportunities per unit can be the most difficult to determine; the lower the opportunity count ,the more critical it is to be accurate.
  • At times, the number of units going through certain process steps is not in the usual proportions due to various business factors. For example, you may be expecting to shut down a portion of your operation for maintenance, so you ramp up the process prior to the shutdown. Disproportionate unit counts can produce inaccurate results for the overall performance measures. Minitab allows you to account for this by entering expected counts for each component, which you should do if any of the counts for a step or component are significantly different (proportionally) from their norm (either too many or too few).
  • The Six Sigma Product Report does not use data recorded over time to show process stability like Minitab's process capabilities (binomial and Poisson). The major difference is that the Six Sigma Product Report allows you to enter data for multiple process steps (either continuous or discrete) and combine the data into a single overall capability analysis, while the other two capability reports depict capability for a single step (with attribute data only) over time.
  • The YTP column in the Six Sigma Product Report calculates the throughput yield for each step/component. Throughput yield is the probability of correctly performing the step or building the component with no defects the first time (no scrap or rework). It is the opportunity-level DPMO raised to the power of the number of opportunities per unit.
  • The YRT column in the Six Sigma Product Report calculates the rolled throughput yield for each step/component. Rolled throughput yield is the probability of correctly performing the step or building the component the number of times specified in the Complexity column, with no defects, the first time (no scrap or rework). It is the YTP raised to the power of Complexity.
  • The Six Sigma Product Report displays the overall YRT (rolled throughput yield) for the entire system. Overall rolled throughput yield is the probability of completing all of the steps or producing all of the components necessary to complete one unit of finished product or service with no defects the first time (no scrap or rework).
  • The bottom line of the Six Sigma Product Report displays combined performance metrics. The combined metrics are:
    • Total defects – the sum of all defects for the steps/components with discrete data
    • Total opportunities – the sum of all opportunities for the steps/components with discrete data
    • Total DPMO – the total defects divided by the total opportunities, then multiplied by 1,000,000. This calculation includes only steps/components with discrete data. The total DPMO is the defect rate, at the opportunity level, for the entire system of steps/components. Note: Total DPMO in the Six Sigma Product Report is the same as the average DPMO in the Capability Rollup Report.
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