Here we show how to use InterpolatedUnivariateSpline to interpolate the same data as before using a 4th order spline, find a (3rd order) derivative, find the roots of that derivative, and then use that information to identify the extrema  for the original interpolating spline. For corresponding examples using UnivariateSpline, see the reference manual page.
Let us assume we have imported the data as before and are now ready to interpolate the data. Interpolation is again a two-step process: construct an interpolating function, then apply it to generate new data values.