Decibel (decibel45) wrote,

When too little knowledge goes a long way...

So apparently there's a lot of people out there who have heard that 'statistics has no memory' and mistakenly think that you never ever multiply probabilities together. This is from a post that catherine81 mentioned in #alg yesterday about some chick who thinks she might have gotten pregnant (the post has since been deleted). I started looking at the odds of getting pregnant after multiple years of having sex using different methods of birth control. Of course this means you take the effectiveness rate of whatever you're using and exponentiate to however many years you're interested in. So something that's 80% effective, if you have sex for five years, it's effectiveness over the entire 5 years is 0.8 ^ 5. Not a hard concept if you think about it for a bit... in order to not get pregnant, you have 'roll the dice' in your favor 5 years in a row.

But I got about 10 people going off about how statistics have no memory and that's just wrong. Apparently they don't understand the concept of calculating the probability of something happening x number of times in a row.

Our school system at work.

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