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» Got Questions? Get Answers. » SCARLETEEN CENTRAL » Sex Basics and Sexual Health » Math Geniuses, I Need You!

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Author Topic: Math Geniuses, I Need You!
Juniata
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Ok folks, here's the deal:
I'm thinking about birth control, and I want to use backup. But math's not my thing, so I'm asking you [Razz]

I'm thinking of using condoms and a diaphram together.

Using conservative statistics--If male condoms have a 15% failure rate and diaphrams have a 20% failure rate, what's the probability that both will fail during a single use?

Thanks!
Juniata

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TheFlash
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Well, I don't know about being a genius, but my math teacher says he's never met someone as gifted as I am since he's been teaching and he's pretty old.

So, I'll try...

If condoms have a 15% failure rate and diaphrams have a 20% failure rate, that means the sperm must get past both. Meaning if neither fails or one fails, there's no pregnancy. Another way to say the same thing is that both must fail for pregnancy. This means you multiply the two together and that is (0.15 x 0.20 = 0.03) or 3% failure rate.

But there's some problems here that make a big difference. First they both must fail and not make a difference if you use them together. So, for example, assume wearing a condom doesn't change the way a diaphram works (or doesn't work). I don't know if that's true, but makes sense they should work together okay. That's a question for someone else.

The failure rates are yearly and you said for a single time. So, assume you have sex every day of the year, you have to divide the answer by 365 (0.03/365=.0000821...) or .008%, or roughly 1 out of every 12,500 times you have sex.

But I think your numbers are wrong. I think condoms fail much less often, but I could be wrong. I have no idea about diaphrams. You can plug and chug the correct numbers if you have them.

Do I get an 'A'?

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-Lauren-
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(Woo, I'm impressed! Nice work!)

What Juniata gave were the typical use rates, which means that the method is not always used or is used correctly.

Condoms and diaphragms are a great pair, and compliment each other well. The perfect use rate for condoms is 2%, and the diaphragm w/spermicide 6%. What say ye, number-cruncher?

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TheFlash
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Thanks Lauren,

So, back to the math. If we assume "Perfect Use" is 2% and 6% as you describe, then 0.02 x 0.06 / 365 = 0.0003...% or roughly less than 1 in every 300,000 times you have sex. Now that's a lot of sex. So, you can see that's a big difference from the 1 in 12,500. I guess that means "perfect use" makes a big difference.

This assumes the failure rates are based on people having sex every day of the year. These are failure rates on a yearly bases right? I don't think people have sex every day when they do the tests? If you tell me how often the people in the statistics have sex, I could figure that in too and come up with a better number.

For example, if the data was collected from people having sex once a week on average, then it's 7 times more likely than I described (daily sex). If that's the case, it would be roughly 1 in every 47,619 times you have your weekly sex.

These are all averages so, "Your mileage may vary." [Smile]

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Heather
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Whoah, the Flash!

I'm off-duty today, but in passing through, just saw this. Might there be any way I might bug you to help us out in making a list of the effectiveness rating of a few different combined methods? It'd be seriously helpful, and very appreciated. We could really use your skills!

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Heather Corinna, Executive Director & Founder, Scarleteen
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Juniata
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Thank you TheFlash. What an appropriate name! I really appreciate your help!

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I LOVE SCARLETEEN. This site is better than sliced bread!

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americangirl
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condoms don't have a 15% failure rate. They are 98% effective. That is a 2% failure rate....
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TheFlash
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Hi Juanita,

Glad you like it. Pretty simple math but you have to make a lot of assumptions which may or may not match the way the statistics were gathered. I may know math, but I don't know the details on how they test birth control, so it's hard to give details without assumptions.

Regardless of the assumptions, comparing the two I'd say in general you are quite a bit better off using 2 methods if and when they can work together without causing trouble. If two methods interfer with each other, then you could make things worse. For example, I'm told 2 condoms make it more likely they will rub together and then both break. I assume this means using a female condom and a male condom together is a bad idea. Sounds though like condom and diaphram work good together. Also, as you can see from the numbers, using them correctly makes a really big difference.
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Hi Heather,

That's very nice of you to say. If you think I can help, sure, if I have time. I'm really not an expert and I don't know much about the sex. If you ask me a question, I can try to answer it. As I mentioned, I would really need to know what the numbers mean in order to say something helpful.

I'm not very good at .html, but maybe you could make a web page where you check off birth control options. Put in a slider that varies between average use and perfect use failure rates. Then give some final statistic about how likely you are to get pregnant given you check more than one together. If you check off two methods that are incompatible, you could show a warning (or make it so you can't pick that one).

Of course if you pick abstinance you can set the odds to zero no matter what additional birth control you use, unless you fill in your name as the Virgin Mary. Then apparently anything can happen. <That's me making a joke FYI [Smile] >

My teacher is always impressed with my abilities with math. He wrote my mom a nice letter telling her he's never seen a student like me in something like 20 years as a teacher. My friends call me a freak show - I won't mention what my enemies call me.

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Leabug
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(sexualgurl- 15% is the typical use failure rate for condoms, whereas 2% is the perfect use failure rate. TheFlash has got the right numbers, no worries!)

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Lea

Posts: 2332 | From: Canada | Registered: Mar 2006  |  IP: Logged | Report this post to a Moderator
   

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