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Thread: Please help - Assumptions with 2 tail t-tests

  1. #1
    Oct 2009

    Please help - Assumptions with 2 tail t-tests

    Hey new to this forum and looking for some help with this stats question as it is slowly sending me round the bend?

    any help would be welcomed!

    How effective is washing your hands in eliminating bacteria? A researcher tests four
    dierent methods - washing with water only, washing with regular soap, washing with
    antibacterial soap, and spraying hands with an antibacterial spray. For 32 mornings she
    washed her hands according to one of the four methods (8 mornings with each method).
    She then placed her hand on a sterile media plate designed to encourage bacteria growth.
    Each plate was incubated for 2 days at 36C, after which the number of bacteria colonies
    were counted. The data is shown in the table below.
    Alcohol Spray, Antibacterial Soap, Soap, Water
    69, 66, 16, 134
    18, 112, 106, 145
    20, 62, 138, 82
    32, 67, 66, 113
    59, 95, 84, 71
    66, 75, 75, 165
    47, 111, 81, 97
    41, 24, 120, 158

    Using JMP, how do you check whether the assumptions for this test (2 tail t-test) are met???
    Last edited by LisaJay; Oct 19th 2009 at 02:29 AM. Reason: over use of symbols sorry
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  2. #2
    MHF Contributor matheagle's Avatar
    Feb 2009
    This looks more like a test of equality between 4 treatments.

    $\displaystyle H_0:\mu_1=\mu_2=\mu_3=\mu_4$


    $\displaystyle H_0:\mu_i\ne\mu_j$ for some $\displaystyle i\ne j$

    I would not do a t-test, nor a bonferroni either.
    IF you're asking for test of constant variance (that's cochran's test)
    or normality (that the kolmogorov-smirnov test)
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