The true defective ppm (not DPMO) for the six sigma level is 0.002, not 3.4 DPMO.
In Six Sigma, it is a common practice to consider that 3.4 DPMO corresponds to a six sigma level, which is a misconception because it assumes a 1.5 sigma shift, which in turn, a mature manufacturing industry process would never accept. In other words, the 3.4 DPMO corresponds to a 4.5 sigma level considering only one side of the specification.
The true defective ppm (not DPMO) for the six sigma level is 0.002, assuming:
1. Process is centred at nominal specification (nominal is better),
2. Two-sided specification
3. Individuals normally distributed
4. Stable process (no special causes of variation).
Charts using 3.4 DPMO for the six sigma level should not be used in practice.