What does 3.4 defects per million mean?

It is the defect rate usually quoted for a Six Sigma process. It assumes the process mean drifts by 1.5 standard deviations over the long term, so a “6 sigma” process behaves like one with 4.5 sigma between its mean and the nearest specification limit. Without the shift, 6 sigma would correspond to about 0.002 defects per million.

What counts as an opportunity?

Each distinct way a unit can be defective that you check for. For a medication administration audited against the five rights, there are 5 opportunities per dose. Define opportunities before you collect data and keep the definition fixed, or sigma levels will not be comparable over time.

What is the difference between DPU and DPMO?

Defects per unit (DPU) divides defects by units, so a unit can count more than once. DPMO divides defects by the total number of opportunities and scales to a million, which lets you compare processes of different complexity.

Free tool · CPHQ® exam topic

Six Sigma calculator

Enter the defects you found, the units you checked and the opportunities for a defect in each unit. Get DPMO, yield and the sigma level.

Result

Sigma level (with 1.5σ shift): 4.25
Defects per million opportunities (DPMO): 3,000
Yield (defect-free opportunities): 99.70%
Defects per unit (DPU): 0.015

3,000 defects per million opportunities is a sigma level of 4.25, using the conventional 1.5 sigma shift.

Formulas

DPMO = defects ÷ (units × opportunities per unit) × 1,000,000.
Yield = 1 − defects ÷ (units × opportunities).
Sigma level = z(yield) + 1.5, where z is the standard normal quantile and 1.5 is the conventional long-term shift.

Worked example

A medication safety audit checks 1,200 administrations against the five rights, so there are 6,000 opportunities. Auditors find 18 defects: DPMO = 18 ÷ 6,000 × 1,000,000 = 3,000, a yield of 99.70% and a sigma level of 4.25.

Sigma levels at a glance

Sigma level	DPMO	Yield
2	308,538	69.1%
3	66,807	93.3%
4	6,210	99.4%
5	233	99.9767%
6	3.4	99.9997%

Sigma levels summarize defect rates, but they do not show whether a process is stable. Track the same measure on a control chart to see special-cause variation, and use an FMEA to decide where defects would do the most harm.

Test yourself: a CPHQ-style question

Which metric is used to quantify process variability in Six Sigma?

Mean
Standard deviation (sigma)
Median
Range

Show answer

B. Standard deviation (sigma)

In Six Sigma, the sigma level counts how many standard deviations fit between the process mean and the nearest specification limit. A higher sigma level means less defect-producing variation.

Questions like this come up in the Performance and Process Improvement domain of the CPHQ® exam. The 249 questions in the app each come with a written explanation.

FAQ

What does 3.4 defects per million mean?: It is the defect rate usually quoted for a Six Sigma process. It assumes the process mean drifts by 1.5 standard deviations over the long term, so a “6 sigma” process behaves like one with 4.5 sigma between its mean and the nearest specification limit. Without the shift, 6 sigma would correspond to about 0.002 defects per million.
What counts as an opportunity?: Each distinct way a unit can be defective that you check for. For a medication administration audited against the five rights, there are 5 opportunities per dose. Define opportunities before you collect data and keep the definition fixed, or sigma levels will not be comparable over time.
What is the difference between DPU and DPMO?: Defects per unit (DPU) divides defects by units, so a unit can count more than once. DPMO divides defects by the total number of opportunities and scales to a million, which lets you compare processes of different complexity.

Sources

Coskun A, Ialongo C. Six Sigma revisited: We need evidence to include a 1.5 SD shift in the extraanalytical phase of the total testing process. Biochem Med. 2020

Checked against these sources on October 29, 2026. For education and CPHQ® exam preparation only, not a substitute for your organization’s policies, official measure specifications or professional judgment. Calculations run in your browser; nothing you enter is sent anywhere.