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deeply philosophical question


philstudent1991
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so I thought I'd ask some smart people to weigh in on this....

 

The "typical use" efficacy rate of condoms allegedly hovers around 80%. This means that out of 100 times, 20 times you will get pregnant, right?. Or, out of 5 times, you will get pregnant once. Since many sexually active couples have sex at least five times and many only use condoms...and by those numbers their chances of getting pregnant is 100%.....why does it not happen more often? Why do people even buy condoms? I'm not understanding this at all...what am I missing?  

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so I thought I'd ask some smart people to weigh in on this....

 

The "typical use" efficacy rate of condoms allegedly hovers around 80%. This means that out of 100 times, 20 times you will get pregnant, right?. Or, out of 5 times, you will get pregnant once. Since many sexually active couples have sex at least five times and many only use condoms...and by those numbers their chances of getting pregnant is 100%.....why does it not happen more often? Why do people even buy condoms? I'm not understanding this at all...what am I missing?  

 

You're missing Jesus, buddy.

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Underdetermination of statistical models....? I guess if you really did a thorough frequency distribution of the entire condom using population you might come up with a figure like 80 percent as regards efficacy, but that probably says little to nothing about the sexual activity of some given individual. All that aside, I'll take those odds all day long.

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so I thought I'd ask some smart people to weigh in on this....

 

The "typical use" efficacy rate of condoms allegedly hovers around 80%. This means that out of 100 times, 20 times you will get pregnant, right?. Or, out of 5 times, you will get pregnant once. Since many sexually active couples have sex at least five times and many only use condoms...and by those numbers their chances of getting pregnant is 100%.....why does it not happen more often? Why do people even buy condoms? I'm not understanding this at all...what am I missing?  

Most birth control failure rates are based on a 12-month period. Condom effectiveness rates also vary greatly based on 'correct use' versus typical use; if you're using condoms correctly, it's much closer to 90-95%. So, 80% efficacy rating means that 20 out of 100 women who use condoms for an entire year will become pregnant. I don't how frequently this assumes that you have sex, but the risk of pregnancy for each occurrence of condom use is much lower than 20%

Edited by perpetuavix
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I think you should read the original research. The report says that a condom is considered to be in typical use because the woman thinks she's using that method.

More importantly, a woman is typically fertile 6 days, usually about 5, out of every 28 days. Knock off seven of those days for, let's call it hygiene reasons, and slightly more than half of the time people have sex she wouldn't get pregnant anyway, no matter what. Add to those numbers the odds of an an egg getting fertilized per cycle in the first place (30%) and the odds of a fertilized egg implanting--a requirement in order to become pregnant (22%), then pregnancy at any point of the cycle, even with no birth control, isn't a sure thing.

This is not to say that pregnancy is a difficult state to achieve with a normal, unprotected sex life (it's not). However, pregnancy isn't a simple conditional. Not a true statement: If woman's internal reproductive organs exposed to sperm, then will definitely get pregnant.

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So one problem that folks to this point have noted is that you're taking these statistics as probability per use, whereas they are probability per year of 'typical' use.

 

But you still might think: 'well, ok, but now wouldn't this mean that the probability of getting pregnant with 'typical' condom use over 5 years is 100%'

 

So here's the other thing that's going wrong. You're thinking of the situation analogously to this, perhaps: 

 

I have a bag with 5 balls, 1 blue and 4 red. Each time I reach into the bag, I remove one ball and do not put it back in the bag. Over 5 pulls, the probability that I pull the blue one is 100%. 

 

But this isn't right, because the probability that someone gets pregnant on any given year is (roughly) independent to any of the other years. So it's rather analogous to putting the ball that you pull out each time back in the bag.

 

(Another way of realizing that the probability can't be 1 over 5 years: probability of getting heads on a coin is not 1 over two flips.) 

 

How do we compute the probability for this? Let's say over five years.

 

P(x) + P(~x)=1 

 

That is, the probability of x (getting pregnant) plus the probability of ~x (not getting pregnant) is 1.

 

So:

 

P(x)= 1-P(~x)

 

What is P(~x) for the five year span? Well, like we said above, it's like pulling out one of the five balls and replacing it each time for five times. So the probability of ~x (=pulling a red ball = not getting pregnant) is just a straightforward independent events situation, where we multiply. So it's (.8)^5.

 

So P(x)= 1-(.8)^5, or about .67. 

Edited by flybottle
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So one problem that folks to this point have noted is that you're taking these statistics as probability per use, whereas they are probability per year of 'typical' use.

 

But you still might think: 'well, ok, but now wouldn't this mean that the probability of getting pregnant with 'typical' condom use over 5 years is 100%'

 

So here's the other thing that's going wrong. You're thinking of the situation analogously to this, perhaps: 

 

I have a bag with 5 balls, 1 blue and 4 red. Each time I reach into the bag, I remove one ball and do not put it back in the bag. Over 5 pulls, the probability that I pull the blue one is 100%. 

 

But this isn't right, because the probability that someone gets pregnant on any given year is (roughly) independent to any of the other years. So it's rather analogous to putting the ball that you pull out each time back in the bag.

 

(Another way of realizing that the probability can't be 1 over 5 years: probability of getting heads on a coin is not 1 over two flips.) 

 

How do we compute the probability for this? Let's say over five years.

 

P(x) + P(~x)=1 

 

That is, the probability of x (getting pregnant) plus the probability of ~x (not getting pregnant) is 1.

 

So:

 

P(x)= 1-P(~x)

 

What is P(~x) for the five year span? Well, like we said above, it's like pulling out one of the five balls and replacing it each time for five times. So the probability of ~x (=pulling a red ball = not getting pregnant) is just a straightforward independent events situation, where we multiply. So it's (.8)^5.

 

So P(x)= 1-(.8)^5, or about .67. 

 

Weird. Is anyone else seeing the same thing I'm seeing?

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This is probably what solves the problem.

 

I'm surprised I got such insightful comments haha

 

perpetuavix is right about the 12-month thing. Just to emphasize, though, "typical" condom use is what is typical for a couple that uses condoms as their primary method of birth control, not using a condom in a typical way every time you have sex—it includes sometimes not using a condom. 

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perpetuavix is right about the 12-month thing. Just to emphasize, though, "typical" condom use is what is typical for a couple that uses condoms as their primary method of birth control, not using a condom in a typical way every time you have sex—it includes sometimes not using a condom. 

 

It doesn't include sometimes not using a condom. It includes sometimes using a condom incorrectly.

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