Three Sigma vs. Six Sigma: Differences, Similarities and Examples

It’s a quality assurance truism that you can’t inspect quality into a product. For a statistician, the difference between three sigma and six sigma is a difference in the degree of deviation from a standard. For a business, these deviations represent lost time, materials, customer satisfaction and profit. The concept of Six Sigma was pioneered and trademarked by Motorola Corp.

Level of accuracy

The biggest difference between the two Sigma levels is the degree of accuracy between outcomes. Three Sigma allows for a greater number of defects per million

defects per million
In process improvement efforts, defects per million opportunities or DPMO (or nonconformities per million opportunities (NPMO)) is a measure of process performance. It is defined as. A defect can be defined as a nonconformance of a quality characteristic (e.g. strength, width, response time) to its specification. › wiki › Defects_…

, whereas Six Sigma requires near-perfect accuracy. This means that many companies consider anything below Six Sigma to be unacceptable.

What is Six Sigma (Difference BtW 6 and 3 Sigma)

What is Six Sigma?

Six Sigma is the highest level of Sigma, which dictates that the margin of error can be up to six standard deviations from the mean. This results in a 99.999997% rate of accuracy, maximizing efficiency and reducing defects more than the lower Sigma levels. This high percentage of accuracy results in roughly 3.4 errors or defective parts per million. These explicit criteria make Six Sigma an essential metric for measuring and controlling product quality.

What is Three Sigma?

Three Sigma is the third Sigma level, which dictates that there can only be a margin of error up to three standard deviations from the mean. This means that 93.3% of all outcomes fall within this range of accuracy. All Sigma levels measure the maximum number of allowable defects per one million parts. For Three Sigma, this means there can be an error rate of three parts per million, or 66,800 defective parts.

While Three Sigma is an effective quality assurance method for various manufacturing processes, some operations require a higher level of accuracy. In instances when higher accuracy is necessary, manufacturing companies implement Sigma levels four, five and six.

Three Sigma vs. Six Sigma

Consider some of the significant distinctions between Three Sigma versus Six Sigma:

Level of accuracy

The biggest difference between the two Sigma levels is the degree of accuracy between outcomes. Three Sigma allows for a greater number of defects per million, whereas Six Sigma requires near-perfect accuracy. This means that many companies consider anything below Six Sigma to be unacceptable. Three Sigmas rate of accuracy is more common among manufacturing companies that are still new in the industry. Startup companies may also progress through each Sigma level until growth and development lead to Six Sigma levels of accuracy.

Strategy and application

As Three Sigma represents half the level of accuracy that Six Sigma dictates, companies implementing this process are often within a median level of growth and development. This means that strategies for improving accuracy and performance can differ from the strategies that companies apply when using Six Sigma.

For instance, a mid-level company that produces digital applications may consider strategies that fix errors or bugs in the app to improve user experience and generate more revenue when users subscribe. Applying principles from Three Sigma requires an error rate of 66,800 application defects per one million downloads. Applying Six Sigma means there are only 3.4 application defects or bugs per one million downloads.

Process vs. methodology

Another distinction between each Sigma level is the difference between the Three Sigma processes and the Six Sigma methodology. In Three Sigma, the focus is on each process that results in an outcome, along with the predictability of each outcomes accuracy rate. Therefore, companies currently progressing through the third Sigma level may establish strict performance and productivity objectives to measure the progress of each process in the manufacturing cycle.

In contrast, Six Sigma serves as a methodology that team members apply to build a collaborative environment where all staff members have equal accountability for their assigned tasks during production. While Six Sigma also supports efficiency in performance and operations, it focuses more on engaging teams through all stages of manufacturing to ensure near-perfect results.

Similarities between Three Sigma and Six Sigma

Despite the differences, both levels of Sigma overlap in several areas, including:

Quality control

All Sigma levels provide frameworks for monitoring the quality of performance for the manufacturing and collaborative processes that occur throughout production. Most companies strive for Six Sigma levels of quality and performance as they achieve growth and success. The emphasis on quality control and progress monitoring means Six Sigma also encompasses and builds on many of the Three Sigma principles.

Statistical approaches

Three Sigma and Six Sigma both rely on statistical evaluations for calculating quality and production accuracy. Both metrics represent error rates that depend on the standard deviation of both levels of accuracy. While the standard deviation changes between Sigma levels, its necessary to calculate this statistic to determine the rate of accuracy between each level of Sigma. Additionally, both levels consider the predictability of outcomes occurring.


The goals for both Three Sigma and Six Sigma are also similar, although strategy integration and performance objectives can differ. In both Sigma levels, companies aim for high levels of accuracy, quality and performance. Therefore, Three Sigma may establish goals to increase the number of viable products and decrease the number of defective products, as Six Sigma builds on these goals to produce higher quality.

Besides performance and quality, both levels of Sigma also focus on effective collaboration and communication between departments. As companies begin to integrate Six Sigma, these goals can often expand from improving individual processes to developing inclusive team environments.

Examples of Three Sigma vs. Six Sigma

You can also calculate the number of allowable defects for both Sigma levels for production numbers under one million. Simply multiply the corresponding level of accuracy for each Sigma level by the total number of completed products to get the number of errors you can have during manufacturing:

Three Sigma example

Suppose a company that manufactures mobile devices is progressing through Three Sigma after developing a new component. The company currently has an output capacity of 275,000 units. Assuming a 93.3% accuracy rate, the company is allowing for up to 18,425 defects with 256,575 products that meet quality control standards. As this value represents a high margin of error relative to the standard deviation, the company may establish short-term objectives that maximize efficiency and support involvement for all departments. With these considerations, the company focuses on goals to improve quality and performance to advance at each Sigma level.

Six Sigma example

Assume the previous example company achieves a Six Sigma level of quality and accuracy when manufacturing the new mobile device components. In addition to these improvements, the company can also produce 780,000 components due to its growth. Assuming an accuracy rate of 99.999997%, the company can expect only 0.0234 parts of a component to experience defects with 779,999.9766 products meeting quality control standards. This represents a near-perfect accuracy rate, which is essential to aim for when implementing the Six Sigma methodology.


What percentage is 3 sigma?

Two sigmas above or below would include about 95 percent of the data, and three sigmas would include 99.7 percent.

What is a 3 sigma process?

What Is a Three-Sigma Limit? Three-sigma limits is a statistical calculation where the data are within three standard deviations from a mean. In business applications, three-sigma refers to processes that operate efficiently and produce items of the highest quality.

Is Six Sigma plus or minus 3 sigma?

Common probability dictates that 99.73% of all outcomes will fall within 3 standard deviations of the mean (above or below); this is Three Sigma. For most purposes, this is a fine system. There are very few scenarios in daily life or most professions where 99.73% certainty or accuracy is not acceptable.

What is the difference between 3 sigma and 4 sigma?

3 Sigma: 66.8K errors per million (93.3% accuracy). 4 Sigma: 6.2K errors per million (99.4% accuracy). 5 Sigma: 233 errors per million (99.97% accuracy). 6 Sigma: 3.4 errors per million (99.999997% accuracy).

Related Posts

Leave a Reply

Your email address will not be published.