• The expression is the equation for expressing the 95% confidence limit around the mean (See for example Statistical Methods 7th Edition – Snedecor and Cochran – pp51-53) . If you have an infinite sample size the 2 will become 1.96 but, unless you happen to know the exact sample size for the mean (which you don’t) the usual practice, for an…[Read more]

  • One control chart for an entire call center isn’t reasonable. You are making no allowances for differences in call type, call difficulty, operator experience, staffing differences (time of day, weekends, skill sets, etc.) and so on.

    If you want to lump everything together and ignore all of the above you could do this just to see what you…[Read more]

  • Feel free to calculate per product but you can calculate per shift just fine. 2 months, 1 week ago

  • Check out something referred to as the 1.5 sigma shift. There has been much debate on this site as to the source and validity of the concept. It will explain why the traditional 3.4 defects per million definition of 6 sigma shows up in the Z table as 4.5 sigma and not 6. 2 months, 2 weeks ago

  • Congratulations ce_seek. You’re welcome. I’m glad I was able to be of some assistance. 3 months, 1 week ago

  • As an analogy….once you reach your desired weight after a diet, do you stop weighing yourself? No. You keep monitoring your weight to be sure it doesn’t change. The same would be true if you reach your desired blood pressure because you are taking drugs to help you. You would still periodically take your blood pressure to make sure nothing…[Read more]

  • If you have zero defects, then you wouldn’t need it. It would be hard to imagine zero defects over a long period of time if you show a lack of control for the short amount of time captured in your chart.

    If you had a Cpk and Ppk of 2+, then go elsewhere first.

    Please be aware that specification limits aren’t used for control chart…[Read more]

  • the takt time is different for each process. reference the formula you have in mind. 3 months, 4 weeks ago

  • No, you do not need a minimum of 30 samples – Gossett put that bit of nonsense to rest back around 1908 or so. Other than the fact that the basic Cpk calculations require data normality there is no connection between 30 samples and normality. Take what you have and press on with the calculation. If you are really concerned about the structure of…[Read more]

  • You can do a capability analysis on attribute data. Check Chapter 9 – Measuring Capability with Attribute Data in the book Measuring Process Capability by Bothe – you can probably get the book through inter-library loan. 5 months, 2 weeks ago

  • I’m not trying to be rude or offensive but the short answer to your question is – start over.

    1. You will need to provide more information concerning the QA score – what is it and how is it quantified?
    a. Is this some kind of personal assessment by yourself – i.e. a 1-10 Likert score based on perception or is it something that is…[Read more]

  • I can’t imagine doing SPC on amount in and amount out if you are looking at quality as your pain point. Knowing nothing else of your process, why not look at quality % with a run chart. If you’re going to do SPC, I bet you’d have too many things to investigate if quality goes out of control. 6 months, 1 week ago

  • Are you sure you did the comparison correctly? Did you wait until the mean and/or standard deviation looked at a stable process reading before comparing the means or sd? 8 months ago

  • Something else to consider – are you sure the issue is one of just confirming a known error rate of 8% and not one of knowing, with a high degree of certainty, if the error rate has changed from say 8% to 9%? If it is the latter then you would need about 30000 samples to confirm this kind of shift with 80% power and an alpha of .05. 9 months, 1 week ago

  • If the defects are actually random and are randomly distributed throughout the year then, from time to time, you could take a random sample of 50 invoices and check to see if you have approximately 4 errors. In order to determine how often you should take a random sample you should take the data from the last year (we are assuming it is in time…[Read more]

  • I have no idea what a .mpj file is but it won’t open in anything I have – try saving it as an Excel file and maybe I or someone else might be able to offer some advice. 10 months ago

  • You will need to provide more detail with respect to dies and rotary indexing stations before anyone can offer much in the way of advice. Is this a situation where there are X number of dies with 4 cavities each in Y number rotary indexing stations or are dies and rotary indexing stations two completely different and separate things that can be…[Read more]

  • The short answer to your question is there aren’t any tests like that. In order to make a meaningful comparison of any kind you need context. Comparing one single number with another single number is a comparison devoid of context. The only thing you can say about any observed difference between two single numbers is a difference exists.

    The…[Read more]

  • Is there any way of getting this done? – Yes, start over because, based on your description of what you are doing, you are a very long way from being able to make any kind of decisions concerning employee performance.

    Let’s do a recap of your initial post.

    1. You said,”We convert the survey responses into continuous data (ex.: Liker scale 1 to…[Read more]

  • @michaelcyger great response….. though I’m way behind on the praise! 1 year ago

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