Objective: To re-examine data on sperm counts over time from 61 studie
s from around the world. Design: Parametric analyses and flexible nonl
inear models of the relation between sperm counts and time. Main Outco
me 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, bu
t a world-wide decline has not been demonstrated. Rigorous assessment
of statistical models should be done before conclusions are drawn. Fle
xible smoothing models are a useful addition to currently available an
alytic methods. (C) 1997 by American Society for Reproductive Medicine
.