[jthacker]

The 99.6% effectiveness statistic comes from the planned parenthood website and I'm assuming they quoted it from this study since they do not cite their source http://humrep.oxfordjournals.org/content/22/5/1310.short

99.6% is the method-effectiveness (i.e. the efficacy if used properly) or 0.4 unintended pregnancies per 100 "women years" (13 cycles)

Other interesting results from the study: - the method-effectiveness rate found for this method is comparable to oral contraceptive - 9.2% stopped using the method due to dissatisfaction - Couples that had intercourse during the fertile period had an increased pregnancy rate of 7.5% - This study was done in European countries and the pregnancy rate was lower than similar studies performed in developing countries