What Is Six Sigma? Methods, Projects and Learning Routes

What is Six Sigma? It is an approach to making a process more consistent. A team defines a problem, checks what the process does and tests changes against evidence. It is also a subject taught in belt courses. But the method and a provider's certificate are different things. A course or certificate does not guarantee a job or higher pay.

Six Sigma focuses on variation and defects: how much a process differs from what is required, and why. DMAIC gives a sequence for work on an existing process. Begin with a useful problem and reliable data. A belt title can describe training or responsibility. But the issuer's assessment rules determine what a given award means.

What Six Sigma means

Variation means that a process does not produce exactly the same result each time. Some variation is expected. The question is whether it matters to the person who uses the result. A form can arrive at different times yet still meet its deadline. An incorrect account number may be a defect regardless of speed.

A defect needs a definition. Without one, two staff members may count the same case differently. A team studying incomplete applications should agree which fields are required and at what point completeness is checked. That makes the measure repeatable.

ASQ's Six Sigma reference describes the method's focus on data and reducing variation. Statistical tools can help a team distinguish a pattern from noise. They do not remove the need to know the process, verify records and define what counts as acceptable.

You can begin by defining a problem and gathering reliable data. A headline defect rate alone does not show which part of a process needs work. For a beginner, the practical question is whether a team can define and measure a problem well enough to study its causes.

How DMAIC works

DMAIC stands for Define, Measure, Analyze, Improve and Control. Each phase answers a different question. Moving through them without evidence can turn a project into a set of completed forms instead of a better process.

Define: agree the problem → Measure: establish the baseline → Analyze: check causes → Improve: test a change → Control: monitor the result.

In Define, agree the user need, process boundary and problem statement. Decide who owns the process. A scope such as all office mistakes is too broad to measure well. A defined application step is more manageable.

In Measure, decide what to count and how. Check that the data reflects the same rules across cases. A large spreadsheet is not by itself a reliable baseline. Missing timestamps or mixed definitions can make it unsuitable.

In Analyze, examine possible causes. A process map and conversations with staff can suggest explanations. Use evidence to check them before calling one a root cause. A reason repeated at a meeting remains a hypothesis until it is tested.

In Improve, try an appropriate change on a controlled scale. Compare it with the baseline using the same definitions. In Control, agree ownership and a way to detect later problems. A good result during a short trial does not prove the process will stay stable without follow-up.

An illustrative process example

Consider a training office receiving applications. Staff report that applications often return for correction. The team wants to understand the problem before replacing its software.

The first task is to define incomplete. Does the application lack a required item when submitted, or is it still incomplete after a staff check? The team chooses one point in the process and writes a clear rule. It also decides which application types are in scope.

Next, it records cases under that rule. The team checks a sample together to see whether different staff classify it consistently. If they disagree, the measurement method needs work before any defect total means much.

Possible causes include unclear instructions, an optional-looking required field or different document rules for different applicants. Those are hypotheses. The team might compare case types and inspect the form to find evidence for each.

A trial could use a clearer instruction for one application type. The team would compare the same completeness measure before and during the trial. It would also check for side effects, such as more time needed to complete the form. A process change is not by itself useful because one measure moves in the preferred direction.

The control plan would name the owner and the check to repeat. It would say what happens when errors recur. The example shows a sequence of questions, not a business case claiming savings, faster service or a defect reduction.

What the example needs before a conclusion

The office would need matching cases and a record of other changes during the trial. If a new request type arrived at the same time as the revised form, the team would need to account for that difference. A before-and-after picture is not by itself evidence of the form's effect.

It also needs a check that the output still meets the user's need. A more complete form may take longer to fill in. The team should decide which trade-offs are acceptable before calling the trial useful. That decision belongs with the process owner and the people affected.

The practical skill is asking what the evidence can support. A team can report an observation, explain a limitation and plan the next check. It does not need to invent a precise gain to make the investigation worthwhile.

Belt levels and responsibilities

Belts commonly describe levels of knowledge or project involvement. A Yellow Belt may contribute process knowledge and data. A Green Belt may run a smaller project with guidance. A Black Belt may handle more demanding analysis and coach others. The exact scope varies by workplace and issuer.

There is no universal ladder that every provider applies in the same way. ASQ's named credentials have their own rules. Its Certified Six Sigma Yellow Belt (CSSYB) has no education or experience prerequisite. Its Certified Six Sigma Green Belt (CSSGB) requires three years of full-time paid work in relevant areas, with no education waiver. Sources: ASQ Yellow Belt and ASQ Green Belt, Checked November 2026.

Read the belt responsibilities guide before choosing a level. Use the Yellow Belt guide for introductory participation or the Green Belt guide for the more involved route. The name of a belt does not by itself show project experience.

Lean and Six Sigma

Lean examines how work flows and where effort fails to help the customer. Six Sigma examines variation and defects through a defined measurement approach. They can be used together when the process has both kinds of problem.

For example, an application might wait in a queue and also contain inconsistent data. Removing a handoff may address the wait, while the data issue needs its own cause analysis. The two questions should not be blurred into one vague efficiency claim.

See how Lean Six Sigma combines the methods for a worked sequence. Use Lean or Six Sigma if you are choosing the starting approach for a given process.

Training versus certification

Training teaches material. Certification is an award issued under a named body's rules. A course may prepare you for a separate exam, include its own assessment or issue a completion record. Those arrangements are not equivalent.

ExpertRating's relevant course is introductory learning with its own certificate. It does not award ASQ certification. The certification comparison separates the issuers, and the training comparison separates learning formats from external award rules.

If your target is ASQ, use the ASQ-specific route to check entry rules before buying preparation. A course provider's willingness to enroll you is not the credential issuer's approval of your experience.

Project management is a related but different subject. A project plan organises the work. It does not replace evidence about the process problem. Compare the project management routes if you need that broader planning foundation.

More in this guide

Start with belt levels to understand responsibility. Compare Yellow Belt and Green Belt if you have a given level in mind. Then choose training or check certification issuers, including the ASQ requirements.

The combined Lean method and method comparison help when your main question is the process itself. Return to the career and short-training comparison when deciding whether this subject fits your wider plans.