ns() function in R can generate the natural cubic spline basis matrix. I checked the reference but still do not know how to calculate such matrix by hand.
For example, I have x = (-2.377, 0.029, 0.106, 2.184). Using ns function with df =3, I can get:
[1,] 0.0000000 0.0000000 0.0000000
[2,] 0.2534264 0.5250116 -0.2672809
[3,] 0.3119701 0.5053060 -0.2565989
[4,] -0.1665931 0.3393594 0.8272338
attr(,"degree")
[1] 3
attr(,"knots")
33.33333% 66.66667%
0.029 0.106
attr(,"Boundary.knots")
[1] -2.377 2.184
attr(,"intercept")
[1] FALSE
attr(,"class")
[1] "ns" "basis" "matrix"
Can anybody tell me how to calculate each value in the matrix?
Thank you so much.
