Dear Maintainer,
Thanks for the excellent package splines. I am writing this email to request you
to consider a suggestion I have with regards to the function ns.
While trying to rework an example from a textbook, I couldn't call ns with
appropriate arguments to reproduce the results. The package documentation also
couldn't help me find the problem. Finally, I found a stack exchange
question
(https://stats.stackexchange.com/questions/588769/natural-splines-in-r-with-ns)?
which helped me understand the problem - the default values of boundary knots
are not useful. The problem is described in the stack exchange question, which I
request you to kindly read.
My suggestion is to change the default value of the argument Boundary.knots to
NULL and calculate its values from? the extreme values of the argument knots
inside the function body if it is NULL and otherwise to keep whatever numerical
value it is assigned at call.
I think it is more intuitive to the user to specify one set of knots assuming
that its minimum and maximum values would be used as the knots beyond which
regression would be linear rather than to know that the function automatically
calculates boundary knots which are not appropriate and so he needs to override
them.
Just so that I am clear, an example. Assume that my variable *alcohol*? has
values from 1 to 100 and I want to specify natural splines at knots 20,40 and
60, expecting linearity would hold below 20 and above 60. Currently, I have to
specify knots as 40 and boundary knots as 20 and 60 as?ns(alcohol , knots =
c(40), Boundary.knots = c(20,60))
If I incorrectly assume that correct values of boundary knots are calculated by
default, I will specify knots as 20,40 and 60 as ns(alcohol , knots =
c(20,40,60)) and get incorrect values for boundary knots as 1 and 100. (I have
done this and the stack exchange post shows that I am not alone).
Hope my suggestion will be considered,
Thanks,
ajith
