Return a spline approximation of the change in log odds.
spline_variablec(varName, x, y, w = NULL)
| varName | character, name of variable | 
|---|---|
| x | numeric input (not empty, no NAs). | 
| y | numeric or castable to such (same length as x no NAs), output to match | 
| w | numeric positive, same length as x (weights, can be NULL) | 
spline y prediction