The last stage of a DMAIC project is control. During the control phase, teams build monitors that let them ensure the process continues to work successfully after changes are implemented across the regular business process. At the same time, Six Sigma teams work to transition the process back to the process owner.
Up until this point in the DMAIC process, Six Sigma teams have worked with statistical analysis tools, and a Black Belt or other Six Sigma expert has been present to walk team members through analysis and interpretation. While many organizations train process owners and other employees in Six Sigma fundamentals, it isn’t always true that a process owner and his or her team will be familiar with the statistical controls that Six Sigma experts have been using. Because of that, appropriate documentation via a control plan and education regarding tools such as control charts might be necessary to ensure business teams can maintain a process and identify when it is out of control and needs remediation.
Revise FMEA
At this time, Six Sigma teams might want to revisit the FMEA tool originally introduced in chapter 13. Six Sigma teams initially use the Failure Modes and Effect Analysis to identify potential failures in a process and causes of those failures. In chapter 13, we discussed how the FMEA listed potential failure points and ranked them according to severity, occurrence, and detection, calculating a total risk priority number.
At the end of the improve phase or beginning of the control phase, Six Sigma teams should revisit the FMEA, noting what recommended actions were completed and recalculating risk priority numbers for the improved process. There are two reasons for revisiting the FMEA. First, the team is able to see that positive and significant changes have occurred because of the solutions adapted during the improve phase. For any root cause that matched a solution implemented, the team would hope to see a smaller risk priority number. Second, an updated FMEA helps the team identify the next problem or root cause that might be addressed. Remember, Six Sigma is a continuous improvement initiative. The team might have implemented a solution and met an improvement goal, but further improvements can always be made. Control is a time to review the process and suggest possible improvements for future projects.
Create a Control Plan
To facilitate continued success, Six Sigma teams should create a written control plan for the process owner. The purpose of a control plan is to help the process owner and business team track and respond to key performance indicators so that the process remains improved. The control plan should be a concise, easy-to-reference document that tells the business team when to monitor, how to monitor, what range of data is acceptable to the monitor, and how to respond with corrective action if the range measured is not acceptable.
Control plans can be spreadsheets, specialized digital documents, or hard-copy documents posted at a
work station. Common elements of a control plan include:
- Company, division, or department name
- Name of person who created the plan
- Date the plan was created
- Name of the person who last edited the plan
- Date the plan was last edited
- Project and/or process name or identifier
- Process owner
- List of process steps where control action is required
- CTQ or metric associated with each action required
- Limit specifications, or the acceptable range of measurements
- The unit of measurement
- The method of measurement
- The necessary sample size
- The frequency of measurement
- The person responsible for measurement
- Where the information is recorded
- Correction actions
- Associated policy and procedure documents
In discussing quality in chapter 8, we introduced the example of a company that makes chocolate bars and noted that the amount of sugar in the chocolate bar recipe was critical to the customer’s experience with the end product. If a Six Sigma team were tasked with improving customer satisfaction with a new chocolate bar product, they might have implemented a solution that ensures the proper amount of
sugar is added to the mixture at the right temperature to incorporate the ingredient appropriately. A control plan for the new chocolate bar solution might look something like the document below.
The above example control plan provides instructions for two specific steps in the process with easy-tounderstand measurement and monitor requirements. To reduce the chance of errors, the Six Sigma team has even specified a special measuring tool for measuring the sugar in the test batch so that every operator performing the monitor measures using the same tool.
At the end of the control document, the team provides steps for corrective action. The first step can be corrected by the operator, who has the ability to calibrate the machine him or herself. The temperature calibration in this case can’t be performed by the operator, which means the process has to be stopped so that someone can attend to the issue. Note that it is always preferable, when possible, to build corrective action at the process level, such as was done with the sugar measurement. This minimizes downtime, puts employees more in control of the processes they own, and helps employees stay involved with the quality process.
The control plan above assumes that manual measurements must be taken or recorded. Optimally, Six Sigma teams should look for ways to automate measurements, which means data can be continuously gathered and converted into statistical process controls such as control charts. Automated data gathering doesn’t mean a control plan isn’t necessary, it just means that a control plan won’t include instructions for gathering the data. Instead, employees and process owners can be instructed to review automated data or control charts and take action if necessary.
You’ll also note that the specification requirements given above are provided with LSL and USL. LSL is the lower specification limit and the USL is the upper specification limit. These are the upper and lower limits of the acceptable range.
Visual Management
In addition to providing a control plan, Six Sigma teams can implement specific visual controls in a workplace to help business teams maintain a controlled process. Some of these tools were covered in chapter 4 on Lean process management, including 5S. Other visual controls teams might implement include signs, posted matrixes and instructions, auditing boards that let teams keep track of individual or group performance over time, color coding, and safety signals. Standard operating procedures can often be distilled to visual representations on posters. A coffee shop, for example, might provide employees with a visual representation of what ingredients are used to create various complex drink flavors. Such a poster ensures that employees can prepare drinks quickly while reducing errors in ingredient inclusion. Other SOP visualizations might include safety procedures in a medical environment, such as visual reminders for hand washing and short pictorial representations for how to operate equipment such as hospital beds. In an office environment, pictorial instructions are found on copy machines, where pictures indicate how paper should be loaded and visual gifs are often displayed on LED screens to help employees remove jammed paper. These are some examples that Six Sigma teams can follow when creating documents that will help business staff accept ownership of an improved process and maintain the improvements made.
SPC Charts
One of the most common methods Six Sigma teams use to monitor a process is the control chart. A number of types of control charts exist, and Six Sigma experts must choose the right control chart for the type of data and analytical purpose. Now, we’ll cover the visual tests that let a Six Sigma team or process owner know that a process is out of control.
A basic control chart has the following elements:
- A line chart of data with plot points for specific data points
- An x-bar line representing the average of the data points
- Lines above and below the x-bar line representing 1, 2, and 3 standard deviations from the
median in either direction - An upper control limit (UCL) line at 3 standard deviations above the median
- A lower control limit (LCL) line at 3 standard deviations below the median
Above is an example of a control chart. The middle line, which is black, is the x-bar line. The x-bar line is bounded by green lines on both sides, indicating 1 and 2 standard deviations away from the mid-line. Those lines are bounded by orange lines on either side: at the top, the upper control limit, and on bottom, the lower control limit. Between the various lines are areas of the control chart, designated as C, B, and A going in either direction. These distinct areas are important for understanding if a process might be out of control. They are also called Zones 1, 2, and 3.
A control chart is best displayed using an automated reporting system or dashboard, where process owners or responsible employees can view it as needed. If automated data collection and control charting is not possible, then a business analyst can be tasked with collecting data and presenting it in this format periodically, though periodic graphical analysis is less likely to catch a problem of control within a process.
Statistical Process Control Tests: Control Charts
Eight tests exist that can quickly tell someone viewing a control chart if a process is out of control.
- Test one: A single point on the control chart appears outside of the upper or lower control limits. If this occurs, process owners should take immediate action, because it is evidence of a major problem within a process. While there is a very remote possibility that shifts outside of three standard deviations can occur randomly, the likelihood is only 3 in 1,000.
- Test two: Nine points in a row appear on one side of the center line. This indicates that a change occurred in the process; if the process owner knows what change caused the shift and it was intentional change, nothing needs to be done and the control chart will right itself over time with the new data. Otherwise, the process owner should investigate the process.
- Test three: Six points on the control chart increase or decrease in a row, indicating the process is becoming less or more efficient or is generating fewer or more errors. Process owners should investigate unless there is a known reason for the trend.
- Test four: Fourteen points on the control chart in a row alternate moving up and down. This could indicate variation in machines, employees, shifts, or over correction.
- Test five: Two out of three points in a row on the control chart are in the upper A section or in the lower A section. This might indicate some type of special cause creating sudden high variation.
- Test six: Four out of five points in a row on the control chart are located in the upper B section(or beyond) or the lower B section (or beyond). This can indicate a problem of major causation or a shift problem similar to that of test four.
- Test seven: Fifteen points in a row are located within the C section above or below the centerline. This can indicate that control limits are no longer relevant to the process; if a team has improved variation of a process, they should recalculate control limits to new parameters. Alternatively, this might occur temporarily when short-term variation is high or low relative to the rest of the points on a control chart.
- Test eight: Eight points in a row on the control chart are located on either side of the center line, but none are located in the C section above or below the line. This could indicate an issue of mixed resources or processes; a team might think they are measuring a single process when they are actually measuring two process, for example. Alternatively, it could indicate a major difference in processing for two employees or teams.
When it’s possible to create control charts and display current data on a regular basis, these charts make
a good addition to a control plan. Individuals don’t have to be well-versed in statistical process control
to learn about the eight tests, and business teams with the benefit of control charts can spend more
time working on production or correcting issues and less time collecting and documenting
measurements.
Control Versus Capability
It’s important to note the distinction between a controlled process and one that is capable of meeting customer requirements. We touched on this concept in unit 1: controlled processes don’t have a lot of variation. Capable processes don’t have a lot of variation and the outputs center around a customer requirement. This is why both control limits and specification limits are important.
Consider the example used earlier in this chapter about sugar in the chocolate mix. The specification limits ranged from 4.9 to 5.1 cups of sugar in each batch. It’s possible for a control chart to show that the process is in control if the measurements range from 3.5 to 3.6 cups of sugar per batch, but the process owner should know that those measurements aren’t going to contribute to a product that meets critical to customer quality requirements.
To understand how a process is performing against specification limits, Six Sigma teams can calculate sigma level and process capability.
Sigma Level
Sigma level is the number of standard deviations between the current process center, as measured by the median, and the nearest specification limit (not control limit.) The equation for sigma level is the smaller of the following calculations:
For example, if a process has an USL of 5 and a LSL of 3, a standard deviation of .25 and a median of 4.2, then you would calculate from the USL, since the median is closer to the USL.
Process Capability
Process capability is calculated by dividing the sigma level by 3. In the case of the example, the capability
is 1.06.
Process capability is denoted as Cpk. A process capability of 1.33 is equal to a sigma level of 4, which is what most experts agree is the minimal level at which most customers will be satisfied. Under statistical process control, many organizations aim for a process capability of 2.0 with minimal acceptable process capability at 1.5.
Team Celebration and Reflection
When Six Sigma teams deem improvements and the related process to be capable and in control, and they’ve passed those processes back to business and process owner control, they should take time to celebrate and reflect on the outcome of the project. This is usually done following the final tollgate review with a sponsor or champion, and can be a quick meeting to close loose ends, recognize the work done by the team, and discuss lessons learned within the process.
The celebration and reflection meeting is also a great time for team members to bring up ideas for possible improvement projects. While improvements – and the related problems and causes – are still fresh on the team’s minds, they can effectively brainstorm ideas for next steps. As with any brainstorming session, no idea should be automatically held off the table because it seems silly, would be too hard to implement, is too costly, or seems too big. Not all of the ideas will become future projects, but the team’s input provides valuable information that the Black Belt can later share with Six Sigma leadership panels.
Control Tollgate Checklist
o The team has calculated the performance and capability of the new process
o The team has written a control plan and communicated it to the process owner
o The team has created a monitor for the process, either through procedures for manual data
collection or automated generation of control charts
o The team has provided the process owner and business team with all tools and information
required to maintain improvements
o The sponsor, champion, or executive leadership has been informed about the state of the
improvements
o The team met to reflect on the project and generate a list of ideas for future improvements