Control Charts; Confused

U chart is chart of choice if the number of parts being tested varies from time period to time period. Otherwise a C chart is fine.

Hey, stop making it complicated. You are starting to sound like that idiot Stan. A C or U chart is fine.

According to the poster: “track the defects per unit”That’s sort of the definition of the C or U chart. Of course this assumes that the defect occurrence is not frequent and that he is seeking to monitor change over time. Also assumes that he is testing some number of units per time period and wishes to see if the process is getting better or worse or staying the same.

You need to ask why you want a control chart. Conrol charts don’t track data, data collection systems do that.
If you know your defect distribution is not predictable, whay aren’t you working on that instead? When defect distribution is not predictable, solutions are usually pretty easy and straight forward.

As explained in ever SPC book in existance, c and u charts have extremely limited usefulness. In electronics, the number of defects found in any given unit is going to very small as compared to say, defects per 100′ of a seemless tube, or defects per 1000 sq ft of float glass (good applications of c and u).
Why would you need a chart when you can tell the folks that the odds of having x or more defects in any given unit is very small and when you have one, set it aside for detailed analysis. You don’t need a chart, you need a simple rule.
The other side of this is he already knows his defect distribution is unpredictable, and any reasonable data collection system will have enough information in it to go solve that problem. In electronics, this unpredictability is often caused by the repair techs skill or a flawed fault tree or a MSA problem.
c ad u assumes that the defects follow a poisson distribution, he already knows that is not true, so he should make it true before worring about a chart.

DPMO is rubbish.  Forget it.

I’m surprised no one has suggested that you simply gather the failure information over a defined period of time and prepare a pareto analysis of the data.  Using a pareto chart has worked best for me when several different failure modes are present in one system test.

Trudy,
While a pareto chart can point you in a direction there is no statistical value behind the chart.
Many false trails can be obtained by using an incorrect chart.
Very obvious trends can be observed and in some cases that may be enough but usually you want to know more than what a pareto chart will offer.

Ron,
I agree.  Pareto analysis may help take the confusion out of what type of chart to use by allowing the focus to be on a single failure mode.  I’ve never applied a control chart for systems using multiple failure modes. 
Trudy