a character string specifying the type of spline basis. "b-spline" for B-spline basis,
"tp-spline" for truncated power basis and "tpi-spline" for monotone (increasing) truncated power basis.
Knots
the internal breakpoints that define the spline used to estimate the NLL effect. By default there
are none.
Degree
degree of splines which are considered.
Intercept
a logical value indicating whether intercept/first basis of spline should be considered.
Boundary.knots
range of variable which is analysed.
Keep.duplicates
Should duplicate interior knots be kept or removed. Defaults is FALSE, which removes
duplicate knots with a warning if duplicate interior knots are found.
outer.ok
logical indicating how are managed x values outside the knots. If FALSE,
return NA, if TRUE, return 0 for the corresponding x values.