Figure 1. Pooled summary of contraceptive efficacy from two WHO male contraceptive efficacy studies (14, 15) where contraceptive failure rate (pregnancy rate) is plotted against the current sperm concentration in the ejaculate. This illustrates a summation of all data pooled from both studies. Data comprise monthly observations of the mean sperm concentration (averaging monthly sperm counts) and whether a pregnancy occurred in that month or not. Pregnancy rate (per 100 person-years, Pearl index) on the y-axis is plotted against the cumulative sperm density (in million sperm per ml) indicating that contraceptive failure rates are proportional to sperm output. The inset is the same data re-plotted in discrete sperm concentration bands rather than cumulatively. For comparison, the average contraceptive failure rates in the first year of use (33, 407) of modern reliable contraceptive methods are indicated by diamond symbols.