Goodmorning,
This is a documentation related question about the B-spline function in R.
In the help file it is stated that:
"df degrees of freedom; one can specify df rather than knots; bs() then
chooses df-degree-1 knots at suitable quantiles of x (which will ignore missing
values)."
So if one were to specify a spline with 6 degrees of freedom (and no intercept)
then a basis with 6-3-1 =2 internal knots should be created. However this is
not what happens:
> library(splines)
> s1<-bs(women$height, df = 6,deg=3)
> s2<-bs(women$height, df = 6,deg=2)
>
> attributes(s1)$knots
25% 50% 75%
61.5 65.0 68.5
> attributes(s2)$knots
20% 40% 60% 80%
60.8 63.6 66.4 69.2
>
i.e. basis is created with an extra knot i.e. bs() chooses df-degree internal
knots
The documentation of "ns" states that: " .... ns() then chooses df - 1 -
intercept knots ..." suggesting that the spline functions create the basis with
df-degree internal knots if no intercept is specified but df-degree-1 internal
knots if the caller explicitly asks for an intercept.
> s1<-bs(women$height, df = 6,deg=3,intercept=T)
> s2<-bs(women$height, df = 6,deg=2,intercept=T)
>
> attributes(s1)$knots
33.33333% 66.66667%
62.66667 67.33333
> attributes(s2)$knots
25% 50% 75%
61.5 65.0 68.5
Is it possible to change the documentation of these functions to reflect their
actual behaviour. For example something like the following:
"df degrees of freedom; one can specify df rather than knots; bs() then
chooses df-degree-1 knots at suitable quantiles of x (which will ignore missing
values) if the intercept argument is TRUE and df-degree if intercept=FALSE."
Christos Argyropoulos
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