Message-ID: <23737FAA-7C14-4605-B9EA-746C73F0CDF8@oulu.fi>
Date: 2016-01-11T14:00:06Z
From: Jari Oksanen
Subject: NMDS axes scores
In-Reply-To: <56939C64.8020009@senckenberg.de>
> On 11 Jan 2016, at 14:13 pm, Bob O'Hara <bohara at senckenberg.de> wrote:
>
>
> Whilst I'm filling bandwidth, I'm not sure Jari's suggestion that you need the interaction term is correct. If a model is linear in axis1 and axis2, then any rotation is also linear, i.e. the transformation is c_1 axis1 + c_2 axis2. So you only need Y ~ axis1 + axis2. Basically, it's still plane. The interaction adds a curve: if you want that you also need to include quadratic terms, i.e. Y ~ axis1 + axis2 + axis1^2 + axis2^2 + axis1:axis2.
>
Me neither: I think my suggestion to include interaction term was wrong. This belief seems to be shared by vegan authors who define linear trend surface without interaction in ordisurf().
Cheers, Jari O.