Objective: To re-examine data on sperm counts over time from 61 studies from around the world.
Design: Parametric analyses and flexible nonlinear models of the relation between sperm counts and time.
Main outcome measure(s): Mean sperm concentrations per milliliter and regression coefficients for possible trends of concentrations over time.
Result(s): A significant decline was found only in U.S. studies.
Conclusion(s): Studies from specific sites have found declines in sperm counts, but a world-wide decline has not been demonstrated. Rigorous assessment of statistical models should be done before conclusions are drawn. Flexible smoothing models are a useful addition to currently available analytic methods.