It's important, no, necessary that new processes be developed with high process capabilities because a process that operates close to a specification limit is an expensive process. If you can develop the process such that it is six sigma capable, you probably won't hear much from it ever again. And that's great.
A colleague and I were reviewing a Cpk study at work recently where the result was 1.2 and the sample size was 25. Note that a Cpk statistic is a point estimate and it has confidence limits. The larger the sample, the narrower the limits. So if we put 95% confidence limits around the calculated 1.2 with n = 25, we get limits of 0.84 and 1.56. That's quite a risky process if it is as low as 0.84.
Sample sizes for process capability studies aren't well understood. I recommend 100+ and the sample needs to be taken over several runs, not all at once. Here's a formula for calculating the confidence interval for a Cpk statistic. If you are responsible for a new process, make sure your calculated Cpk statistics include the confidence limits.
