Sunday, October 05, 2008

99% effective = 1% failure

Sitting here between Masses, I just watched a commercial about some new contraceptive (a plastic thingee) that, whatcha know, promises to help women escape from the difficult task of having to remember to take a pill every day.  It's advertised as being "99% effective".  So what does that mean?

Well, let's run the numbers.  I'm thinking, mathematically, that "99% effective" means, if a woman using the contraceptive has sex once a week (something the "Sex in the City generation" would consider pitiful), that within two years of starting the contraceptive, the law of averages will catch up on her, the contraceptive will fail, and she'll become pregnant.  Of course, this'll happen sooner if she's more sexually active.  Then what happens?

Am I missing something here?  Then again, I'm also the guy who figures that milk that is 2% lowfat still has 98% to account for.

8 comments:

Barb, sfo said...

Those ads drive me crazy. There's a pill out there that is also "99% effective" that also helps clear skin blemishes, according to the commercials (that I'm subjected to several times an hour if I watch TLC or the Style channel--which I often do, with my adolescent daughter.)

When we taught NFP, we figured out that any contraceptive method's effectiveness rate is lowered considerably when you consider that everything is 100% effective during the 3/4 of the month when a woman is naturally infertile. Do the math on THAT one! (I'm no good at math, but NFP works.)

Neuropoet said...

Funny how most people don't seem to understand basic math anymore - is it the schools, or something else?

~Jenny

Phase1976 said...

Hopefully God is laughing at our attempts to take His place instead of shaking his fist at us!

I love to hear stories about babies conceived even though tubes were tied, etc. God's reminder that he is still boss, and loves them so much as to give them a beautiful gift.

Dymphna said...

This sounds like a new version of the IUD. That wonder device left millions of women with various injuries.

Mickey said...

I never thought about it that way! Maybe someone should run an ad pointing this out. Wonder if it could be timed to come on after every birth-control ad?

IngoB said...

Dear Fr. Toborowsky,

the 1% means either the typical-use (including human error in application) or the perfect-use (without it) failure rate. Possibly these are identical here, whereas for a condom they are very much not.

In either case, this number represents the calculation of a "per-year-risk" based on some medical study that considers the number of months (or alternatively menstrual cycles) of women using the contraceptive and compares it to the number of pregnancies that occurred. The frequency of sex any couple in the study may have is not counted and any effects of that are expected to average out over large study populations.

The simplest method of calculation is the Pearl index: For example, you may have 200 women in the study who participate for a total amount of 6000 months (on average 30 months, but some may drop out much sooner and some much later). Let's assume that women only drop out due to pregnancy, and there were 5 cases in spite of using the contraceptive. The one would calculate:

(5 cases)/(6000 months)*(12 months/year)*(100%)=1%.

This means that in spite of using this contraceptive, on average 1 woman in 100 will become pregnant within one year. If you want to "reverse" this, then it would mean that on average one woman would have to have sex for 100 years before she would become pregnant. This number is true in an "average over population" sense no matter how often she actually has sex. (Clearly if she has none, there is no chance. And if she has sex all the time, likely the chance is somewhat higher. But this is likely a broad distribution and for most couples this number will be about right.)

For statistical perspective: taking a look at the number of motorcycles in the US, and the number of fatal crashes with motorcycles per year in the US, I estimate that their "death rate" is about 0.3%. For this I have assume that motorcycles are driven around on average a quarter of the year (bad weather, just a hobby, some don't drive at all...). So this contraceptive is about three times riskier.

Best,
Ingo

P.S.: None of the above is an endorsement of artificial contraception, which I consider morally wrong. But it does not help the discussion if one gets the stats wrong. The general opinion that contraceptives like the pill safely prevent pregnancies is essentially correct.

Maggie said...

Interesting thing is, if your math is accurate ("...within two years of starting..."), a contracepting couple using this device would have a child approximately every 2-ish years.... which is a typical span of child-spacing for NFP-using and ecological-breastfeeding couples.

Of course, that's assuming that the child conceived in that 1% isn't chemically aborted by the devices hormones which make it difficult for a new baby to implant.

As anyone (ought to) know(s), the only 100% fail-safe way to prevent pregnacy is absitence or complete removal of the sexual organs. (I was going to say abstienence until menopause...but Abraham and Sarah and Elizabeth and Zechariah would probably disagree!)

Tony said...

Father,

It just about drives me crazy when I hear that to reduce the number of abortions we need more contraception.

I was explaining my reasoning to my mom.

When there was no contraception, occasionally people slipped up, and maybe became pregnant.

At the advent of contraception, people who were no longer worried about getting pregnant, would have sex much more often.

If condoms are 95% effective, then the law of averages catches up with you after 20 time. If you do it 40 times you're twice as likely to get pregnant using contraception as not.