[R-sig-eco] How do GDM knots relate to spline coefficients?
Greg Guerin
greg.guerin at adelaide.edu.au
Wed Jul 5 05:06:49 CEST 2017
Hello,
I want to identify the sub-intervals of environmental gradients with the greatest species turnover rate from a fitted GDM (gdm::gdm) by calculating rise over run for each spline/segment.
I fitted models with 5 splines (default is 3) and allowed automatic definition of knots (via quantiles). The sum of spline coefficients for each predictor equals the height of the overall function on the y-axis.
However, GDM outputs include 5 knots and 5 spline coefficients per fitted predictor, with the lowest and highest values of the knots representing the extremes of the gradients in the predictor data. I had assumed each coefficient refers to the additional y-axis height attained in each segment/sub-interval along the gradients as defined by knots, but 5 knots only gives four segments (i.e. knot1 function1 knot2 function2 knot3 function3 knot4 function4 knot5).
Can anyone explain how individual splines are related to the knots (i.e. Given same number of knots as coefficients) so that I can work out which gradient sub-interval goes with each coefficient?
Cheers,
Greg Guerin
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