HAVE SPERM COUNTS BEEN REDUCED 50-PERCENT IN 50 YEARS - A STATISTICAL-MODEL REVISITED

Objective: To reanalyze data that were used in a linear model to predi ct that mean sperm counts have been reduced globally by approximately 50% in the last 50 years. Design: The mean sperm counts and their temp oral distribution were reanalyzed via several different statistical mo dels (quadratic, spline fit, and stairstep). Conclusion: There are sev eral reasons why a published linear regression model is inappropriate to infer a 50% reduction in mean sperm counts in the last 50 years. Th ese include [1] the potential selection biases that may have occurred with the 61 assembled studies such that they are not representative of their underlying populations; [2] the likely variability in collectio n methods, in particular, the lack of adherence to a minimum prescribe d abstinence period, as has been stated for the largest study, which c ontained 29.7% of all the subjects included in the analysis; [3] the p aucity of data in the first 30 years of the 50-year trend analysis; [4 ] the fact that if the last 20 years of data are examined, which conta ins 78.7% of all the studies and 88.1% of the total number of subjects , there is no decrease in sperm counts, in fact, sperm counts were obs erved to have increased; [5] the conflicting data from a large individ ual laboratory, which was not prone to the collection variability that likely occurred between the 61 studies, that did not suggest a declin e in mean sperm count or seminal volume during a comparable time perio d, even though this laboratory published the data that were largely re sponsible for the high historical values in the linear model; and, mos t importantly, [6] the variety of other mathematical models that perfo rm statistically better at describing the recent data than the linear model and thus offer substantially different hypotheses. The data are only robust during the last 20 years of the analysis, in which all the models, except the linear model, suggest constant or slightly increas ing sperm counts.