Reversal of sterilization laser laparoscopy and endometriosis. Reply of the authors [letter]

The author expresses agreement with Dr Corsons comment that it would have been helpful for readers to actually see the actual life table data on pregnancy rates following laser therapy of endometriosis. However Dr Corsons comments reflect confusion about the specifics of the methodology used in this study. The data were illustrated both by methods to graphically illustrate cumulative pregnancy rates over time and by mathematical models to fit the observed data. Life tables are the optimal method to illustrate data from clinical fertility trials with variable lengths of follow-up. The data points in this technique involve calculations based on observed pregnancy rates with corrections for those lost to follow-up. This method makes no assumptions about the rate of conception but does assume that patients lost to follow-up do not differ significantly from those who remain in the study. Mathematical models to simulate the observed data points are based on differing assumptions and the validity of the mathematical simulation is based on the validity of the assumptions inherent to the model. Generally the simpler the model the more rigid the assumptions and the poorer the fit to the actual data. The single-parameter Cramer model which assumes a constant pregnancy rate over time and 100% fertility given infinite length of follow-up has been demonstrated to fit poorly data from endometriosis treatment trials.