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A meta-analysis of 61 sperm count studies revisited
1Department of Population Dynamics, School of Public Health, Johns Hopkins University, Baltimore, Maryland 21205, USA. SBECKER@SPH.JHU.EDU
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.