[R-sig-eco] [EXTERNAL] Queries on regression analysis

[Cade, Brian]

While the previous responders have provided some useful advice, it was a
bit misleading.  The linear model for continuous responses does not
automatically assume a normal distribution (of the errors, of which the
residuals are an estimate).  A specific way of estimating the conditional
mean in the linear model assumes a normal distribution of errors.  More
generally, you can estimate the quantiles of the empirical distribution of
the continuous responses with linear quantile regression, which makes no
assumption about a parametric form of the error distribution and naturally
accommodates heterorgeneity.  You can use the median estimate (0.50
quantile regression) as a measure of central tendency rather than the
mean.  But, almost always it is more informative to estimate some interval
of quantiles (say 0.10 to 0.90) to adequately characterize how the response
changes with covariates.   More advanced transformation approaches with
quantile regression will allow you to handle proportions (responses bounded
on [0, 1] interval) and discrete counts.

Brian

Brian S. Cade, PhD

U. S. Geological Survey
Fort Collins Science Center
2150 Centre Ave., Bldg. C
Fort Collins, CO  80526-8818

email:  cadeb using usgs.gov <brian_cade using usgs.gov>
tel:  970 226-9326



On Thu, Aug 8, 2019 at 5:16 AM Chitra Baniya <cbbaniya using gmail.com> wrote:

> We have the data of "gall_diameter" and "elevations". Our objective is to
> see how does gall diameter vary along the elevation gradient. In our case
> the elevation gradient refers to the range between 1500 to 2500 m asl with
> data collected in every 250 m interval.
> Our data did not follow normal distribution. Gall_diameter is a continuous
> dependent variable. Can we apply "glm" to see the relationship between
> gall-diameter and elevation in our case?
> How can we decide the distribution in our data and its family?
> Thank you all in advance for your help.
>
>
> --
>
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>
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>
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> *Chitra Bahadur Baniya, M Sc, M Phil, PhDAssociate ProfessorCentral
> Department of BotanyTribhuvan UniversityKirtipurKathmandu, Nepal*
> orcid.org/0000-0002-8746-7601
>
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