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Help with Sample Skewness

  1. Jul 25, 2010 #1
    Hi everyone,

    I'm using the following formula to calculate skewness

    [tex] g_1 = \frac{m_3}{m_2^{3/2}}
    = \frac{\tfrac{1}{n} \sum_{i=1}^n (x_i-\overline{x})^3}{\left(\tfrac{1}{n} \sum_{i=1}^n (x_i-\overline{x})^2\right)^{3/2}}\ , [/tex]

    However, when I try excel on to calculate skewness I get a different results. For example this set:

    1
    1
    1
    1
    1
    1
    1
    9
    9

    I get: 1.33630621
    Excel gets: 1.619847741

    Is excel using a different formula, or am I doing something wrong? o_O

    Ted.
     
  2. jcsd
  3. Jul 28, 2010 #2

    statdad

    User Avatar
    Homework Helper

    I believe Excel uses this:

    [tex]
    \frac n {(n-1)(n-2)} \sum{\left(\frac{x - \bar x}{s}\right)^2}
    [/tex]
     
  4. Jul 29, 2010 #3
    Any particular reason why
    excel uses a different formula?


    Ted.
     
  5. Jul 29, 2010 #4

    statdad

    User Avatar
    Homework Helper

    I can't really answer why their choice was made. Note that unlike the mean (for example) there are several quantities proposed to estimate skewness, and none is really preferred as "the correct way" to do it.
     
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