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RE: AI-GEOSTATS: In need of some help.

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  • Hugo PILKINGTON
    You could probably try using the Mann-Whitney U test which is a distribution-free (nonparametric) statistical test (I assume you re comparing two means). I m
    Message 1 of 7 , Dec 20, 2000
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      You could probably try using the Mann-Whitney U test which is a
      distribution-free (nonparametric) statistical test (I assume you're
      comparing two means). I'm not to sure about your very small sample sizes,
      but it should be all right.

      Regards.

      Hugo Pilkington
      ____________________________________________________________
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      h.pilkington@...
      Tel: +33 (0)141 79 68 68 http://www.ceses.org
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      -----Message d'origine-----
      De: mercury1@... [SMTP:mercury1@...]
      Date: mardi 19 décembre 2000 18:08
      À: ai-geostats@...
      Objet: AI-GEOSTATS: In need of some help.


      Hi Folks!
      This is my first post to this list. Hope it is not out of place. I need a
      way to compare two small populations (very small sample sizes..5 and
      6....both
      of which lack normality). I would like to compare them based on 3-5
      parameters.
      Because of the above limitations I have given up on the validity of a
      t-test
      (which assumes a normal distribution and larger sample sizes). My basic
      question
      is this: are these two small populations statistically different or do they
      belong to the same population? I have asked many elementary level stats
      folks
      and have not been entirely satisfied with their solutions. So, I pose this
      'problem' to you.
      Thanks for your time.
      Happy Holidays!
      -Harland

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    • Yong Wang
      Mann-Whitney U test does require that the distributions of two populations are similar. It is distribution free but not assumption free. Resampling (or
      Message 2 of 7 , Dec 20, 2000
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        Mann-Whitney U test does require that the distributions of two populations
        are similar. It is distribution free but not assumption free.

        Resampling (or randomization) methods should work. Basically, you will mix
        the two samples, and then randomly separate them into two groups, and
        calculate the parameter you are interested (e.g., average difference);
        repeated the process for a large number of times. You will get a
        distribution of the parameter, compare the parameter estimate from original
        data with this distribution to see how likely you would be able to get the
        original value by chance.

        You can run this through Excel by set up a macro. Or check
        www.resample.com, there is information about the methods and software.

        Best,

        Yong Wang





        At 10:08 AM 12/19/00 -0700, mercury1@... wrote:
        >
        >Hi Folks!
        >This is my first post to this list. Hope it is not out of place. I need a
        >way to compare two small populations (very small sample sizes..5 and
        6....both
        >of which lack normality). I would like to compare them based on 3-5
        parameters.
        > Because of the above limitations I have given up on the validity of a t-test
        >(which assumes a normal distribution and larger sample sizes). My basic
        question
        >is this: are these two small populations statistically different or do they
        >belong to the same population? I have asked many elementary level stats
        folks
        >and have not been entirely satisfied with their solutions. So, I pose this
        >'problem' to you.
        >Thanks for your time.
        >Happy Holidays!
        > -Harland
        >
        >--
        >* To post a message to the list, send it to ai-geostats@...
        >* As a general service to the users, please remember to post a summary of
        any useful responses to your questions.
        >* To unsubscribe, send an email to majordomo@... with no subject and
        "unsubscribe ai-geostats" followed by "end" on the next line in the message
        body. DO NOT SEND Subscribe/Unsubscribe requests to the list
        >* Support to the list is provided at http://www.ai-geostats.org
        >


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      • Tom Nolan
        Harland, You could use the exact form of the Wilcoxon Rank Sum test, which is appropriate for sample sizes of 10 or less per group. Computational details are
        Message 3 of 7 , Dec 28, 2000
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          Harland,

          You could use the exact form of the Wilcoxon Rank Sum test, which is
          appropriate for sample sizes of 10 or less per group. Computational details
          are shown on p. 120 of "Statistical Methods in Water Resources," Helsel and
          Hirsch, 1992, Elsevier. The test is commonly used to determine whether two
          groups are from the same population (i.e. have the same median and other
          percentiles), or alternatively whether the medians are different.

          Tom Nolan

          > -----Original Message-----
          > From: ai-geostats-list@... [mailto:ai-geostats-list@...]On
          > Behalf Of mercury1@...
          > Sent: Tuesday, December 19, 2000 12:08 PM
          > To: ai-geostats@...
          > Subject: AI-GEOSTATS: In need of some help.
          >
          >
          >
          > Hi Folks!
          > This is my first post to this list. Hope it is not out of place.
          > I need a
          > way to compare two small populations (very small sample sizes..5
          > and 6....both
          > of which lack normality). I would like to compare them based on
          > 3-5 parameters.
          > Because of the above limitations I have given up on the validity
          > of a t-test
          > (which assumes a normal distribution and larger sample sizes).
          > My basic question
          > is this: are these two small populations statistically different
          > or do they
          > belong to the same population? I have asked many elementary
          > level stats folks
          > and have not been entirely satisfied with their solutions. So, I
          > pose this
          > 'problem' to you.
          > Thanks for your time.
          > Happy Holidays!
          > -Harland
          >
          > --
          > * To post a message to the list, send it to ai-geostats@...
          > * As a general service to the users, please remember to post a
          > summary of any useful responses to your questions.
          > * To unsubscribe, send an email to majordomo@... with no
          > subject and "unsubscribe ai-geostats" followed by "end" on the
          > next line in the message body. DO NOT SEND Subscribe/Unsubscribe
          > requests to the list
          > * Support to the list is provided at http://www.ai-geostats.org


          --
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        • Syed Abdul Rahman Shibli
          With five to six samples per population, concluding anything from the tests would really be pushing it. Complementing the results with any deterministic
          Message 4 of 7 , Dec 28, 2000
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            With five to six samples per population, concluding
            anything from the tests would really be pushing it.
            Complementing the results with any deterministic
            knowledge of the underlying population (genesis,
            noteworthy features, prior experience, etc) could lend
            some measure of validity to what you will eventually
            conclude from such tests (i.e. do they make sense).

            Unfortunately, doing that often leaves one in the
            unsavory position of realizing that there is more
            uncertainty than first thought of. Somewhat counter-intuitive,
            but so true in my personal experience.

            Syed

            -----Original Message-----
            From: Tom Nolan <btnolan@...>
            To: <ai-geostats@...>
            Date: Thursday, December 28, 2000 10:41 PM
            Subject: RE: AI-GEOSTATS: In need of some help.


            >Harland,
            >
            >You could use the exact form of the Wilcoxon Rank Sum test, which is
            >appropriate for sample sizes of 10 or less per group. Computational
            details
            >are shown on p. 120 of "Statistical Methods in Water Resources," Helsel and
            >Hirsch, 1992, Elsevier. The test is commonly used to determine whether two
            >groups are from the same population (i.e. have the same median and other
            >percentiles), or alternatively whether the medians are different.
            >
            >Tom Nolan
            >
            >> -----Original Message-----
            >> From: ai-geostats-list@... [mailto:ai-geostats-list@...]On
            >> Behalf Of mercury1@...
            >> Sent: Tuesday, December 19, 2000 12:08 PM
            >> To: ai-geostats@...
            >> Subject: AI-GEOSTATS: In need of some help.
            >>
            >>
            >>
            >> Hi Folks!
            >> This is my first post to this list. Hope it is not out of place.
            >> I need a
            >> way to compare two small populations (very small sample sizes..5
            >> and 6....both
            >> of which lack normality). I would like to compare them based on
            >> 3-5 parameters.
            >> Because of the above limitations I have given up on the validity
            >> of a t-test
            >> (which assumes a normal distribution and larger sample sizes).
            >> My basic question
            >> is this: are these two small populations statistically different
            >> or do they
            >> belong to the same population? I have asked many elementary
            >> level stats folks
            >> and have not been entirely satisfied with their solutions. So, I
            >> pose this
            >> 'problem' to you.
            >> Thanks for your time.
            >> Happy Holidays!
            >> -Harland
            >>
            >> --
            >> * To post a message to the list, send it to ai-geostats@...
            >> * As a general service to the users, please remember to post a
            >> summary of any useful responses to your questions.
            >> * To unsubscribe, send an email to majordomo@... with no
            >> subject and "unsubscribe ai-geostats" followed by "end" on the
            >> next line in the message body. DO NOT SEND Subscribe/Unsubscribe
            >> requests to the list
            >> * Support to the list is provided at http://www.ai-geostats.org
            >
            >
            >--
            >* To post a message to the list, send it to ai-geostats@...
            >* As a general service to the users, please remember to post a summary of
            any useful responses to your questions.
            >* To unsubscribe, send an email to majordomo@... with no subject and
            "unsubscribe ai-geostats" followed by "end" on the next line in the message
            body. DO NOT SEND Subscribe/Unsubscribe requests to the list
            >* Support to the list is provided at http://www.ai-geostats.org


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