S Becker1, K Berhane. 1. Department of Population Dynamics, School of Public Health, Johns Hopkins University, Baltimore, Maryland 21205, USA. SBECKER@SPH.JHU.EDU
Abstract
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.
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.
Authors: Rebecca Z Sokol; Peter Kraft; Ian M Fowler; Rizvan Mamet; Elizabeth Kim; Kiros T Berhane Journal: Environ Health Perspect Date: 2006-03 Impact factor: 9.031