Everyone talks about how the 98% prevention rate is over the course of a year. How does this relate to a per use scenario and why do they use the by year rate vs the per use. I feel like the per use would be much lower.
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The effectiveness per use would have to be much higher than the per year rate. Let’s say you have sex 50 times in a year. If condoms work 99.95% (fails 1 time out of every 2000 uses) of the time, they would have a 97.5% effectiveness rate for the year. The math is: .9995^50 =.975
It’s more intuitive to me if you lower the effectiveness rate to 50% per use. Odds of success in 2 uses would be .5 x .5 = .25. Odds of success in 3 uses would be .5^3 =.125. Odds of making it through a years worth of 50 uses approaches 0.
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