system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If birth control is only ~99% effective, are my wife and I statistically likely to get pregnant after having sex 100 times?
gpt
The 99% effectiveness is measured as the chance to get pregnant over an entire year, not per individual encounter. As in, using the pill, perfectly, over an entire year, 1% of couples will get pregnant. 

Additionally, the chances of a particular outcome remain the same (unless the circumstances change). Meaning if you have 100 bags with 10 marbles, five black and five white, the chance of drawing a white marble per bag will always be 50%. That is the same whether you just drew a black marble in the last bag, or whether you just drew 99 white marbles in a row.