title: A meta-analysis of 61 sperm count studies revisited.
abstract: OBJECTIVE
  To re-examine data on sperm counts over time from 61 studies from around the world.
  
  
  METHODS
  Parametric analyses and flexible nonlinear models of the relation between sperm counts and time.
  
  
  METHODS
  Mean sperm concentrations per milliliter and regression coefficients for possible trends of concentrations over time.
  
  
  RESULTS
  A significant decline was found only in U.S. studies.
  
  
  CONCLUSIONS
  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.affiliation: Department of Population Dynamics, School of Public Health, Johns Hopkins University, Baltimore, Maryland 21205, USA. SBECKER@SPH.JHU.EDU
authors.forename: S
authors.initials: S
authors.lastname: Becker
authors.forename: K
authors.initials: K
authors.lastname: Berhane
mesh_terms: D014943:Global Health; D006801:Humans; D008297:Male; D015233:Models, Statistical; D012044:Regression Analysis; D013076:Sperm Count; D018709:Statistics, Nonparametric; D014481:United States