Loading ...
Sorry, an error occurred while loading the content.

Re: AI-GEOSTATS: Log transformation and zeros

Expand Messages
  • Ruben Roa
    ... some 0 values. When I log-trans I have to translate the values by hading some value. I guess you mean you have to do something arbitrary about the zeros
    Message 1 of 8 , Oct 2, 2002
    • 0 Attachment
      >Hi
      >
      >I'm analysing fisheries data (number of fish caught per hour) and I have
      some 0 values. When I log-trans I have to translate the values by hading
      some value.

      I guess you mean you have to do something arbitrary about the zeros before
      the log transform. The delta distribution is a generalization of the
      lognormal for the presence of zeros. See:
      Pennington M. 1983. EFFICIENT ESTIMATORS OF ABUNDANCE FOR FISH AND PLANKTON
      SURVEYS. Biometrics 39:281-286.
      If you are interested, i have template worksheets that compute the
      statistics from delta/lognormal distributions, including confidence bounds,
      by using Land's theory of linear combinations of the normal mean and
      variance.
      You can speak in Spanish to me if you feel more comfortable.
      Saludos
      Rubén
      http://webmail.udec.cl

      --
      * 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
    • Ernesto Jardim
      Hi I was investigating my data and it is possible to identifie areas of zeros on the outside limits of the distribution, so it can be possible to model the
      Message 2 of 8 , Oct 3, 2002
      • 0 Attachment
        Hi

        I was investigating my data and it is possible to identifie areas of
        zeros on the outside limits of the distribution, so it can be possible
        to model the spatial behaviour in two steps.

        My guess is that I can simple reduce the kriging area to leave the zero
        area out.

        My doubt is how to model boundaries. I'm sure this is a common problem,
        so if you can give me some references I'll look forward to find them.

        Thanks and regards

        EJ

        On Wed, 2002-10-02 at 19:33, Donald E. Myers wrote:
        > Adding a constant to all values will shift the distribution but will not
        > change its shape. If the fraction of zeros is large then you will likely
        > not have a lognormal distribution and hence taking logs may not solve
        > the problem. If you intend using kriging (after applying a log
        > transform) then you will have to worry about the bias correction when
        > you re-transform, to do that the theoretical solution requires
        > multivariate lognormality (univariate is not sufficient).
        >
        > You might want to look at the spatial pattern of the zeros, i.e., is it
        > plausible to separate the data set spatially and have most of the zeros
        > in only one region?
        >
        > Donald E. Myers
        > http://www.u.arizona.edu/~donaldm
        >
        > Ernesto Jardim wrote:
        >
        > >Hi
        > >
        > >I'm analysing fisheries data (number of fish caught per hour) and I have
        > >some 0 values. When I log-trans I have to translate the values by hading
        > >some value.
        > >
        > >My question is which value is the best ? is there any works about this ?
        > >
        > >I usually had 1 so that I get values between 0 and infinite (no negative
        > >value) but I have doubts about it.
        > >
        > >Regards
        > >
        > >EJ
        > >
        > >
        > >
        > >
        > >--
        > >* 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
      • Ruben Roa
        ... on the outside limits of the distribution, so it can be possible to model the spatial behaviour in two steps. ... area out. ... if you can give me some
        Message 3 of 8 , Oct 3, 2002
        • 0 Attachment
          >Hi
          >
          >I was investigating my data and it is possible to identifie areas of zeros
          on the outside limits of the distribution, so it can be possible to model
          the spatial behaviour in two steps.
          >
          >My guess is that I can simple reduce the kriging area to leave the zero
          area out.
          >
          >My doubt is how to model boundaries. I'm sure this is a common problem, so
          if you can give me some references I'll look forward to find them.
          >
          >Thanks and regards
          >
          >EJ

          Intrinsic geostatistics, the theory based on random functions, does not
          allow for 'boundary effects'. There should be no interaction between the
          random variable and its field (in practice, no decrease of density near the
          borders). On the other hand, transitive geostatistics, the theory based on
          purposive randomization, does allow for border effects and estimation of
          boundaries, which may fall anywhere between zero and non-zero observations.
          The difference between intrinsic and transitive geostatistics is as basic
          as the difference between model-unbiased and design-unbiased statistical
          inference.
          See
          Petitgas. 1993. Geostatistics for fish stock assessmens: a review and an
          acoustic application. ICES J Mar Sci 50:285-298.
          Petitgas and Lafont. 1997. EVA2: estimation variance. Version 2. A
          geostatistical software on Windows 95 for the precision of fish stock
          assessment surveys. ICES CM 1997/Y:22.

          Rubén
          http://webmail.udec.cl

          --
          * 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
        • Brian R Gray
          while possibly outside the domain of your original question, I suspect that you may be able to treat your yield data as integers by treating the denominator as
          Message 4 of 8 , Oct 3, 2002
          • 0 Attachment
            while possibly outside the domain of your original question, I suspect that
            you may be able to treat your yield data as integers by treating the
            denominator as an offset variable (as a technicality, I'd argue that, even
            after dividing by a constant, your data remain discrete--just not
            integers). this common technique would appear to take you back into the
            discrete world that Nicholas touched on. brian

            ****************************************************************
            Brian Gray
            USGS Upper Midwest Environmental Sciences Center
            575 Lester Avenue, Onalaska, WI 54650
            ph 608-783-7550 ext 19, FAX 608-783-8058
            brgray@...
            *****************************************************************



            Ernesto Jardim
            <ernesto@ipimar. To: Nicholas Lewin-Koh <nikko@...>
            pt> cc: Mailing List AI-Geostats <ai-geostats@...>
            Sent by: Subject: Re: AI-GEOSTATS: Log transformation and zeros
            ai-geostats-list
            @...


            10/02/2002 09:47
            AM
            Please respond
            to Ernesto
            Jardim





            Hi

            The data are not discrete. We collect number per hour, so it's a yield !

            Thanks

            EJ

            On Wed, 2002-10-02 at 15:27, Nicholas Lewin-Koh wrote:
            > Hi,
            > If the data are counts, ie integer number of fish and not tons, you
            > might want to try a discrete model such as a negative binomial or
            > Poisson. I have listed some references below, the top two have a more
            > Bayesian flavor.
            >
            > Nicholas
            >
            >
            > Alexander, N., Moyeed, R., Stander, J. (2000). Spatial modelling of
            > individual-level parasite counts using the negative binomial
            > distribution, Biostatistics, 2000, 1, 453-463.
            >
            > Diggle, P. J., Moyeed, R. A., Tawn, J. A. (1998). Model-based
            > geostatistics (with discussion), J. R. Statist. Soc. C, 47, 299-350.
            >
            > Gotway, C.A., Stroup, W.W. (1997) A Generalized Linear Model Approach
            > to Spatial Data Analysis and Prediction. Journal of Agricultural, Bio-
            > logical and Environmental Statistics 2(2), pp. 157­178.
            >
            >
            > On Wed, 2002-10-02 at 19:24, Ernesto Jardim wrote:
            > > Hi
            > >
            > > I'm analysing fisheries data (number of fish caught per hour) and I
            have
            > > some 0 values. When I log-trans I have to translate the values by
            hading
            > > some value.
            > >
            > > My question is which value is the best ? is there any works about this
            ?
            > >
            > > I usually had 1 so that I get values between 0 and infinite (no
            negative
            > > value) but I have doubts about it.
            > >
            > > Regards
            > >
            > > EJ
            > >
            > >
            > >
            > >
            > > --
            > > * 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






            --
            * 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
          Your message has been successfully submitted and would be delivered to recipients shortly.