system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If condoms have 99% success rate, what causes that remaining 1% to fail?
gpt
That success rate is not per instance. It is an efficacy rate, indicating the percentage of couples who conceived during a period of one year while using condoms as their sole contraceptive method.

The couples that conceive are a combination of people who decided to stop using contraception and people who used condoms improperly and an unlucky few who have condoms break in proper use.

The data is commonly estimated to be poorly representative of the actual effectiveness of condoms due to the fact that many factors contribute to the failures over one year other than condom technology itself, and that with the irresponsible users weeded out in a one year test period the efficacy rate would improve drastically in longer tests.