Fit fertility rates with Schmertmann's Calibrated Spline estimator (Schmertmann 2014). The user must specify the population size and calibration data using a extrainfo string of the form CALIBRATED_SPLINE:PInfo[,CSHandle]. PInfo is a number that specifies the population per age interval in the fertility object. CSHandle is name of the results object with ResultsType="CSDATA" containing the calibration data. If not specified it defaults to "DefaultCSData".
For the HFC build method fertility rates are split into single-year intervals using CALIBRATED_SPLINE, negative rates are replaced with zeros, and all rates are then rescaled so that the total fertilty rate of the original curve is preserved. This is the method used by the Human Fertility Collection.