[R] Documentation of B-spline function

[Christos Argyropoulos]

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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