[R] [FORGED] Re: Help to solve modeling problem with gamm
Rolf Turner
r@turner @end|ng |rom @uck|@nd@@c@nz
Fri Feb 28 23:16:50 CET 2020
On 29/02/20 8:11 am, Abby Spurdle wrote:
>> There was clearly a non-linear growth pattern such that an additive
>> mixed effects model was proposed to model the behavior of biomass
>> as a function of time and treatments.
>
> The presence of non-linearity (in x) does not necessarily mean that
> you can't use a "linear model". In general, linear models work well
> for biological growth curves with small to medium sized data.
>
> Technically, this is off topic, however, the notion of what is
> linear and what is not linear, is a recurring theme on this mailing
> list.
>
>> When plotting the residuals a clear cone-shaped pattern was
>> observed
>
> Based on classical statistics, the obvious approach is to transform
> your data. This is easy to do, however, some expertise is required
> to back-transform parameters/estimates and interpret them.
Although it is indeed off topic I would like to emphasise a point that a
lot of people don't seem to get. A linear model is
*linear in the parameters*
not in the predictors. For example
y = beta_0 + beta_1 * x + beta_2 * x^2 + E
is a linear model. It is linear in (beta_0,beta_1,beta_2). It is not
of course "linear in x".
cheers,
Rolf Turner
--
Honorary Research Fellow
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276
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