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How to express process capability in this case ?

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  • #31193

    Michael Schlueter
    Participant

    Hi,
    Today I found some measurements I made 10 years ago on annual throughput time. At that time we used the annual sigma to define UCL and LCL values for the next year to establish process control on design projects. The mean changed from year to year, of course.
    We did not specify USL and LSL at that time, only targets for the mean. Just for curiosity: is there a way to come up with a resonable Cpk in this case (probably not)? Or an alternative to quantify process capability?
    I do not want to shine up the process capability by inventing suitable spec. limits which did not exist. The ideal target value would have been 0 weeks, anyway, with a variability of 0 weeks: “Ask for a new design – just get it today.”
    I appreciate your comments. Thanks in advance,
    Michael Schlueter

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    #82083

    Chip Hewette
    Participant

    There is an absolute maximum length of time your company desires to spend on a design project.  As you stated, there is no lower limit, aside from the practical space-time continuum boundary.  Some internal customer, perhaps the VP of  Engineering, could set the desired maximum length of time.
    Granted, each project may fit into various types, ranging from breaking fundamental laws of the universe to simple modification of an existing product.  For each type of project, there is a maximum time limit.  There are probably three or four types of projects in your department.
    For that arbitrary USL, based on the needs of the company and its external customers, one can compare past historical project durations.  Chances are some projects exceed the USL.  What proportion of projects exceed that USL?  This can be stated in DPMO.  Although years ago, no one was attempting to meet some USL, the historical data would be quite useful in establishing a benchmark.
    From the DPMO, if desired, one could calculate a sigma level.  In this case, it would seem to be ‘long term.’ and it would also appear that ‘short term’ does not apply.
    If one simply reports the DPMO of project durations (by type of project), the team can set reasonable 6S improvement goals.  One can easily quantify the dollar savings based on the DPMO by applying a cost per unit time to the project.  This cost is likely a weighted average of engineer cost, technician cost, overhead cost, etc.
    From a DPMO and $ point of view, each year’s review of the department would demonstrate the impact of projects.

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