Spline term for prevalence formulas

A b-spline basis for smooth covariate effects, e.g. prevalence = ~ s(day). Matches stm::s() exactly — including the df = min(10, nval - 1) default — so spline-term coefficients agree with stm. (You can also use splines::bs()/splines::ns() directly.)

Usage

s(x, df, ...)

Arguments

x

Numeric predictor.

df

Basis dimension; defaults to min(10, length(unique(x)) - 1).

...

Passed to splines::bs().

Value

A spline basis matrix (with class "s").