[R] mfp package - can I exclude some powers?

[Eleni Rapsomaniki]

Hi,

The mfp package suggests fractional polynomial transformations for
non-linear predictors. For continuous variables that take negative values
the suggested transformations tend to look ugly, e.g. if x1 is the
original variable it is transformed to something like: (x1+5.67)^1 *
log(x1+5.67), where 5.67 is the shift to make it positive.
Can I somehow modify the mfp function to exclude some powers among those
tried (e.g. those involving log and 0.5). I am willing to sacrifice some
precision for a more interpretable transformation.

Or is there another package to do something similar?

Any suggestions very much appreciated!
Eleni

Medical Statistician
Epidemiology & Public Health
University College London, UK