interpolation

"Marwan Khawaja" <mk36 at aub.edu.lb> writes:

I have 15 data points (weight at birth) by age which I want to
interpolate back in time (to 5 more age points).

I'm not sure if I understand what you are asking.  The phrase "back in
time" seems to indicate extrapolation rather than interpolation.

There are many functions in R to do this and I wonder if anyone has
experience in using these -- any preference/caveat etc? I am trying
to find an alternative to linear interpolation.

You could try interpSpline in the splines package.  That produces a
"natural spline" that can be used for interpolation within the range
of the data or extrapolation beyond the range of the data.  Linear
extrapolation, based on the derivative of the fitted function at the
end points, is used.
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