I have two years worth of quality issues/month data and I am trying to calculate the control limits of a process based on this info. I am trying to determine what my monthly target should be (looking at upper control limit only). Should I look at this problem with 2 subsets/time points and 12 subgroups or 12 subsets/time points and 2 subgroups. Calculating the contol limits with each case returns different solution. Any help you guys can offer would be great. Thanks.

I have two years worth of quality issues/month data and I am trying to calculate the control limits of a process based on this info.

I would think with such low frequency (and guessing or hoping low resolution), you might consider I-MR calculations.

it would be helpful to see the data.

it sounds like customer complaint data? what is an 'issue'?
What do you mean by "monthly target"? That is an unusual term in the context of control charts...

Also I think your terminology is a bit off. The subgroup in this case as you have described it is a month. There are 24 months = 24 subgroups.

Note: I (well OK many someones who work for me) make these types of charts all of the time. but I am always leary of home work assignments...so if you could clarify I - and others - could do a better job of answering your question.

As Mr.Bev D said, it is better to see the data.

Why because means as per the querry, it is difficult to understand the data with VW2404 is attribute data or variable data

Regards / Raghu

Thanks for all the responses. Here is the data as requested (see attachment).

As you can see I am still collecting data for the rest of this fiscal year. Based upon this data, I am trying to determine what would be acceptable targets monthly targets for the upcoming year. I was originally thinking that calculating the UCL would be a good approach to this, but I am open to other ideas. Thanks again.

You can try the control limits by using the attribute control chart (C-Chart).

Regards / Raghu

agree that the c chart is the appropriate chart, with the area of opportunity (or the subgroup) being a month. and it should eb a single chart - there is nothing magical about a new fiscal year or calendar year for that matter.

not sure what you are trrying ot do with the goal? aren't goals supposed to be improvements? I would be looking for a sustained improvement in the average count...