Testing for Sphericity in Phase I Control Chart Applications

Gitte Bjørg Windfeldt, Søren Bisgaard

    Research output: Contribution to journalJournal articleResearchpeer-review


    When using (x) over bar -R charts it is a crucial assumption that the observations within samples are independent and have common variance. However, this assumption is almost never checked. We propose to use the samples gathered during the phase I study and the test for distributional sphericity, to check this assumption. We supply a graph of the exact percentage points for the distribution of the test statistics. The test statistics can be computed with standard statistical software. Together with the graph of the exact percentage points, the test can easily be performed during a phase I study. We illustrate the method with examples. Copyright (C) 2009 John Wiley & Sons, Ltd.
    Original languageEnglish
    JournalQuality and Reliability Engineering International
    Issue number7
    Pages (from-to)839-849
    Publication statusPublished - 2009


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