Analyzing FTY

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    James Barrett

    This can’t be a new topic, but I’ve done some searching and can’t find the answers I’m looking for.
    I am trying to analyse the FTY data out of my manufacturing process (aerospace, low volume, manual). If possible I’d like some pointers with where to start. The data is most assuredly not Normal, and naturally there’s a hard limit at 100%, so any on how to interpret control charts, analysis of Pp/Cp values, etc.
    Very broad question, sorry about that.


    Robert Butler

    There is no requirement for data normality with respect to using control charts. – See Wheeler and Chambers Understanding Statistical Process Control 2nd edition pp. 76 Section 4.4 Myths About Shewhart’s Charts – Myth One: it has been said that the data must be normally distributed before they can be placed upon a control chart.

    For calculation of Pp/Cp values for non-normal data see Measuring Process Capability by Bothe – Chapter 8 Measuring Capability for Non-Normal Variable Data.

    For regression analysis – there are no restrictions on the distributions for either the X’s or the Y’s. Applied Regression Analysis 2nd Edition, Draper and Smith pp. 12-28 – or, if you are in a rush – bottom of pp-22-top of pp-23.

    For t-test’s and ANOVA – both are robust with respect to non-normal data. The Design and Analysis of Industrial Experiments 2nd Edition Owen L. Davies pp. 51-56

    You should be able to get all of these books through inter-library loan.

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