I’m like Beetlejuice, except you only have to say ‘Statistician’ once!

I’m like Beetlejuice, except you only have to say ‘Statistician’ once! February 25, 2012

Remember you can vote once per day for the About.com Atheism Awards.  I’m one of five nominees for Best Atheist Blog.  More details here.

The Bright Maidens, a trio of young Catholic women who try to dispel myths and misconceptions about their faith, frequently do themed blog carnivals, and this week their topic is “Instructing the Ignorant.”  Elizabeth Hillgrove of Startling the Day covered a lot of topics in her post, but you can’t be surprised to find out it was this one that caught my eye:

I got to explain to her, no, NFP is not like the rhythm method anymore. I explained that the rhythm method was used back in the 1930s and that we know a lot more about how best to avoid or to achieve pregnancy while keeping God in our bedrooms.

I also used the opportunity to explain that most of the methods are at least 97% effective when used properly (which relies on married couples communicating — another benefit), which is far more promising than something like condoms. The error margin of condom use increases exponentially with every use (statisticians, help me learn how best to explain this, please!), while NFP methods remain effective as long as couples chart and communicate.

You call for statisticians and I appear!

I think I know what Elizabeth is referencing when she talks about the risk of condom failure growing with every use, so here comes the quickie explanation.  Imagine you have a weighted coin that is really, really likely to come down heads every time you flip it.  Let’s say 97% heads and 3% tails.  Obviously, you’ll call heads every time you flip it and make a profit.  But if your goal isn’t to be right most of the time, but to never be wrong, you’re going to run into a problem pretty fast.

Rosencrantz and Guildenstern, flipping coins

How do you figure out how likely it is you’ll lose a coin flip if you keep calling heads.  Well, the chance that you’re never wrong in N flips is .97^N.  So, the chance that you miss at least one is 1-.97^N.  So if we flip 20 times, the odds I’ll miss at least one call is 46%.  Not good if failure is costly.

The coin flip isn’t exactly analogous to the failure rates of condoms.  According to the Guttmacher Institute, with perfect use, condoms are 98% effective over a year of use.  So the errors don’t mount up anywhere near as fast as they would if they were only 98% effective each time you used them.  But the same problem exists — in five years of use at 98% efficacy, 10% of users will get pregnant.

But this problem is decidedly not unique to condom usage.  It applies to any method of forestalling pregnancy where the chance of failure each time you use it is independent.  ‘Independent’ is a statistics word which means the chance of failure is the same in every trial — it doesn’t matter what the previous results have been.  A coin flip is independent, but drawing cards from a deck is not, as long as you’re not replacing the cards you drew.  Most contraceptives’ risk of failure is independent or at least has an independent component (putting a condom on correctly, taking the pill at the same time every day), but IUDs and sterilization are more binary — either the procedures were done right or not — the independent component is small.  NFP presumably has an independent risk — the risk of making errors in charting.

I think the most likely difference between NFP and methods like the Pill and condoms has nothing to do with those ‘perfect use’ statistics.  Those numbers tend to be irrelevant since a lot of people use contraception correctly.  The ‘actual use’ success rates for condoms and the pill over a year are 91% and 83% respectively.  At those rates, the five year failure rate for condoms shoots to 62%.

Guttmacher doesn’t have statistics on ‘typical use’ for NFP, just the perfect use rate of 98% effective.  My hunch is that more people who sign on for NFP hit that mark than people who use condoms.  NFP is visibly hard — people who commit to using it take classes.  Condoms are perceived as easy to use, and abstinence only education doesn’t clue people in on common errors or even that their use is not necessarily intuitive.  So, the people that opt for the ‘hard’ method, aware of how hard it is, are probably more likely to stick to the ‘perfect use’ regimen than people who think they have an easy option.

This difference might very well vanish if NFP became more common, so NFP users were less likely to be people who had made a carefully considered choice.  Ideally, everyone trying to delay pregnancy would put as much effort into their regimen as NFPers do.  Or they’d opt for a method that requires low user commitment like an IUD or a patch.

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  • Nathaniel

    Nice theory, but wrong. NFP has a typical use failure rate of 25%.


    So even with a small committed minority, it fails more often than condoms or the pill. Turns out that avoiding sex at the woman’s most randy time of the month is harder than advertised.

    • Well, that is one side’s propaganda, here’s the other side’s.

      • Nathaniel

        Is it propaganda because you have credible evidence of biased or unbalanced aspects of the linked organization, or is it because you disagree with it?

        • I don’t know about the American Pregnancy Association and what biases it might or might not have. But then I didn’t claim the propaganda necessarily originates there. It could just as well be some lazy intern mixing memories of high school sex ed and half-assed googling and thus unconsciously relaying it.

          What I do know, is that the article you linked to
          – doesn’t cite any sources,
          – downstates perfect-use efficiency to “may reach effective rates around 90%” which is clearly incorrectly low
          – gives an actual use failure-rate of “approximately 25%” comporting with the “25.3%” usually cited for the more general “periodic abstinence” category of methods, which, as the article I linked to explained, can’t be taken as a good estimate for NFP.

          To get one degree less meta, the one actual study that had a respectable sample size in a western country found an actual use failure rate of 1.8%. I’m admittedly uncomfortable most of the times NFP-advocates refer to this result precisely because the sample was self-selected from the, in your words, “small committed minority” already attending NFP classes. So it’s not a number we can generalize without caution. But as to your specific claim it’s a clear-cut refutation.

          • Nathaniel

            Alright. Fair enough. Just realize that “propaganda” is fairly loaded turn to swing around.

          • Andrew G.

            It’s worth noting that a quarter of the participants in that study were using additional contraception in the form of barrier methods (i.e. having protected intercourse while fertile).

          • Worth noting for what message? The success rates were also calculated separately and the differences were insignificant. Nor does it show any violation of ideological commitments, because that commitment didn’t exist in the first place.

            It does show NFP users to able to achieve better success rates at condom use too, but that’s what everyone would have expected in the first place.

  • Andrew G.

    The ‘actual use’ success rates for condoms and the pill over a year are 91% and 83% respectively. At those rates, the five year failure rate for condoms shoots to 62%.

    I think you missed a (1-x) there – 91% over five years gives an effectiveness of 62%, i.e. a failure rate of 38%. (0.91^5 = 0.624)

    (The failure rate doesn’t grow exponentially; the success rate falls exponentially, which is not the same thing.)

    While Guttmacher don’t break down their “typical use” data for periodic abstinence by different methods (and their “perfect use” figures are basically informed guesswork), they do have an overall number for it (and not a good one, either – 25% failure rate). I looked for more detailed info on this and didn’t find any – I do wonder if the low usage rate of NFP in the developed world makes this point harder to research.

  • Andrew G.

    Incidentally, you didn’t seem to explicitly make the key point – that the original Catholic poster may have been right to claim that contraception failure rate increases (albeit not exponentially) with time, but they were dishonest in claiming that the same was not true for NFP.

    To correct further (including correcting my previous correction):

    The ‘actual use’ success rates for condoms and the pill over a year are 91% and 83% respectively. At those rates, the five year failure rate for condoms shoots to 62%.

    The Guttmacher figures are the reverse of what you quoted – 91.3% for the pill and 82.6% for condoms, making the 5-year failure rates 36.6% (pill) and 61.5% (condoms).

    • And wisely so, because Ms. Hillgrove didn’t necessarily make that claim.

      I don’t know her and can’t offer an authoritative interpretation of her writing, but I’ll not that she writes of an “error margin” increasing “with every use” rather than a “failure rate” increasing “with time”. One other way to interpret this would be this:

      In perfect use and given the likely failure modes failure of condom use is failure of a specific (though maybe unidentifiable) condom. The failure probability would then be a function of the number of condoms used. And that function would have the form Leah explained. So the number of unintended pregnancies per unit time probably grows superproportionally with the usage frequency. In contrast to this an NFP failure would be a result of misidentifying the infertile window either through a charting error or a bodily malfunction. As long as it is greater than or equal to one, the number of sexual acts is irrelevant. So basically NFP is scalable and condoms aren’t.

      Now most couples probably don’t want to scale to the frequencies that would make this important. But it’s still a good point for two reasons: First it is a nice tu quoque on the point that NFP is only effective with restrictions on sexual behavior. And more importantly second, scalability is a good sign if you expect the moral law to be beautiful.

      Also, even if Ms. Hillgrove meant something wrong that doesn’t by any means establish dishonesty. There are also honest mistakes.

  • keddaw

    I can’t understand the Catholic Church’s stance on NFP vs barrier contraception – they both use knowledge about what makes someone pregnant in order to try to stop that happening. They both can fail (God’s will) and improper use massively increases this failure rate.

    It gets worse, I read recently on this issue that the ‘success rate’ of NFP is at least partly down to the inability of a fertilised egg to implant towards either side of a woman’s most fertile period, thus leading to the ‘death of many humans’ thanks to NFP that wouldn’t have occurred using a barrier method. Now I personally couldn’t care less about a fertilised egg but the Catholic Church appear to want to hold it up as a whole human being with all the rights and ‘dignity’ that comes with, so supporting a birth control method that actively increases the number of ‘aborted’ fertilised eggs seems counter-intuitive.

    • Alex

      There are pragmatic arguments I am sure: contraception contributes to a culture of casual sex , in which the value of commitment is downplayed. But even in the case of married couples, contraception works against the body’s natural order, it shuts God out of the decision of when to have children, and separates the act of having sex with the willingness to create a new life. By contrast NFP works with the body’s natural order, because it is still open to new life and it respects the fertile periods during a woman’s cycle.

      Don’t ask me to explain what I just wrote though! I’m just giving what I think the stock response is.

      • Patrick

        You’re correct as to what I’ve heard as the stock response.

        But they get SO ANGRY when you point out that this is just saying that NFP is ok because it doesn’t work very well.

        I’m not sure how that contradiction is supposed to be resolved.

      • Yup, that’s it, and it’s not that hard to understand either.

        It’s quite simply the difference between not using one of the body’s natural facilities and actually frustrating it. We look at it at the level of the individual sexual act and ask if any of its natural purposes was subverted. That is obviously the case with a condom. But with NFP every consumation that takes place is already infertile by its nature.

        If you transform it from sex to nurishment that would be the difference between eating low-calory food and bulemia. I think the difference is fairly obvious even though they both use knowledge of what makes someone fat in order to stop that happening.

        Also note that using NFP rather than artifical contraception doesn’t take a couple to the A-OK level, only to the more wishy-washy level of weighting the consequences. And a baby certainly can’t be counted as a bad consequence in itself.

        • Nathaniel

          Birth control is birth control is birth control. All of different methods are attempting stop or retard chance of pregnancy.

          The only difference is that NFP means having less sex, which is why the church likes it. If you don’t want to get pregnant, no fun for you.

          If it truly were about subverting natural processes, then the church would be against diet coke too.

          • Weight control is weight control is weight control. All different methods are attempting stop or retard weight gain.

            The only difference between a diet and bulemia is that dieting means less eating, which is why people like it. If you don’t want to get fat, no fun for you.

            If it truly was about subverting natural processes, then people would be against diet coke too.

        • Alex

          Thank you. Your bulimia comparison was very helpful to me. Obviously this sort of argument rests on one subscribing to a variant of natural law morality – the idea that only actions that fulfill the natural end of a a thing’s purpose are moral.

          Before getting into a meta-ethical discussion though, it seems to me that people do not disapprove of bulimia because they believe that the natural end of the mouth is eating. Rather they disapprove of bulimia because of a whole mess of psychological baggage normally associated with people with bulimia, not to mention the ick factor of forcing oneself to throwing up.

          A more apt comparison would be (hypothetical) diet pills that actually work. Would they be immoral because they frustrate the natural end of the digestive system?

          • Re. diet pills… Ooooh, I like that one.

            — Diet pills are taking a “shortcut” against nature to bring about some end (weight loss)
            — Refraining from eating and increasing exercise would align one’s will with nature and bring about the same end naturally

            I thought of a couple points to mention:
            — One has to eat; one does not have to have sex (just saying it can be harder to refrain from eating “a little too much” when one has to eat regularly to begin with)
            — Obesity is an inherent negative outcome, regardless of preference/desire/view; pregnancy is not inherently negative, but typically situational or based on personal desire/want/preference

          • I’d have some nit-picking to do with your definition of natural law, but since you don’t want to go there I won’t either.

            Actually I think the psychological baggage is mitigating. We tend to assume bulimics aren’t free in their decision to induce vomiting and therefore not culpable. But we wouldn’t ask that question in the first place if we didn’t already know something to be wrong with the behavior. And I don’t think it’s the correlation with other pathological behavior either, because on that standard we would have to disapprove of lots of other things people don’t normally disapprove of. Also, we have a (mostly slanderous) cultural image of the late Roman upper class showing the same behavior at banquets and interpret that as pretty much the stereotypical (though again mostly fictional) example of decadence, i.e. analyze it in moral categories.

            The ick factor gets closer, but then we the obvious next question would be why this particular action is associated with an ick factor. While it isn’t dispositive, I actually think an ick factor is a good prima facie argument for the icky thing being disordered. Often it is us knowing something we haven’t realized on a cognitive level yet. Now things can also icky because instincts paint with a broad brush (like tame rats sharing a category with justifiably icky sewer rats) or because of cultural conditioning (like a racists thinking miscegenation icky) or for developmental reasons (like pre-pubescent children thinking kissing icky). So ickyness is not actual proof of disorderedness. But it is good enough a sign that I would want it explained before just concluding that something icky is perfectly fine.

            As to the hypothetical working diet pills:
            I see a bit of a difference, in that they could also have legitimate medical applications. There are people who literally can’t stop overeating if their life depends on it. Nowadays they get gastric bands and if working diet pills became available it would seem legitimate to have them take those pills. But in that case the body isn’t functioning properly in the first place; it would be more analogous to taking the pill to prevent menstrual cramps, which is actually OK.

            But if someone healthy took the working diet pill simply to separate the nurishing and gustatory purposes of eating, yes I think that would be immoral.

    • Can you tell me where you recently heard the getting worse part?

      If there is data behind that, and if it’s a relevant part of the effect you probably are right that we have to give up on NFP. On the other hand, if it’s on the “drinking coffee” level, that would probably be OK under double effect.

      • keddaw

        Not strictly related to what I had previously read (NB. it could just have been a comment, so don’t take it as fact unless I can dig up an article backing it up!) but here is an interesting one on spontaneous abortions:

      • keddaw

        Another one, from wiki:
        “A related application of the principle of double effect is breastfeeding. Breastfeeding greatly suppresses ovulation, but eventually an ovum is released. Luteal phase defect, caused by breastfeeding, makes the uterine lining hostile to implantation and as such may prevent implantation after fertilization.[26] Some pro-choice groups have expressed concern that the movement to recognize hormonal contraceptives as abortifacient will also cause breastfeeding to be considered an abortion method.”

        All of which is missing the big picture – sex is good, people who engage in it are not bad and just because you believe in some natural* order of things doesn’t mean you get to impose your will on others who disagree.

        * ALL medical procedures defy nature!

        • So? As even Wikipedia understands, this is a question of double effect.

          On the “big picture”:
          – Ah, the good old “if you don’t like slavery don’t own slaves” argument. A classic that never gets old.
          – Nature is a technical term here and doesn’t refer to biology. If you don’t know that you are not informed about the actual arguments. If you do know it you’re arguing it bad faith. No, gay penguins aren’t relevant either.

          • keddaw

            The way you discuss natural implies, ironically, the will of a supernatural being. All medical treatments interfere with that, I don’t see why you’re OK with transplants/antibiotics but not barrier contraception.

            Not sure how slavery gets thrown in here, except as you wish to enslave women into only having sex on your (religion’s) terms.

            Sex is good, sex is fun. NFP prevents pregnancy as does the barrier method and many other forms of contraception. The problem we have is too many people, and I include myself in this, are willing to play mental games and accept your unacceptable premise just to show that you’re still wrong further down the line, but we should be aware that allowing that conversation might make you believe you have won point A (human life begins at fertilisation [when gametes meet, or when sperm enters cell?]) but that is not the case and we should not allow the conversation to later take that as a given.

          • @keddaw: I can’t reply to your comment as we’re threaded as deep as this allows. The issue, as I’ve understood it is that we’re allowed to use reason and technology to improve upon things that were never in God’s plan. Things like disease and organ failure are supposed to have entered the world at the fall.

            Reproduction, however, has always been seen as a good and part of man’s participation in God’s creative power.

            So, you can thwart unintended evils that have been caused by sin and were not part of God’s explicit plan; but you cannot thward intended goods that have been part of God’s purpose all along.

            Does that make sense?

          • keddaw

            @Hendy, “Does that make sense?”

            No! Not even close.

            “are supposed to have entered the world at the fall” But no-one (that one would give a damn about having a conversation with), even the Catholics, believe in a literal fall, so this makes no sense. A ‘Fall’ without a real Adam and Eve descends into speculative metaphorical readings of ancient mistranslated works, that’s no basis for a rational conversation.

            “Reproduction has always been seen as a good…”
            In a western-privileged, rich person worldview perhaps, but not objectively or universally so.

            And the idea that one can thwart an omniscient and omnipotent being’s purpose is so misguided as to be laughable. This is why I can say for sure that some gods cannot exist, but I cannot say for sure that no gods exist.

        • @keddaw:

          Whether or not you care about having a conversation with someone isn’t the criteria you asked for. You wanted to know why [assuming to a Catholic] antibiotics/prosthetics would be acceptable and contraception would not.

          I answered that [within the framework of how Catholics think about things, which is what you wondered].

          Catholics dogma is that the fall was a real event, even if it was figurative (didn’t happen in a garden 6000 yrs ago, etc.). Whether or not all Catholics, or those worth having a conversation with, believe that is also a separate matter.

          Your last point is a fair one. I don’t have the answer. Catholics would probably say something regarding free-will. Holiness is aligning one’s free will with what God wants for you. NFP is aligning your will with God’s natural design; using barrier methods or other contraceptive methods is preventing the natural mechanics of a created good (sex).
          As an aside, sometimes I find commentors respond to me as if I’m Catholic, which can conflate the point of having a conversation. Whether or not you or I think the answer is any good is one thing… whether or not it is consistent within the framework of Catholicism, in my opinion, is another and the only discussion that really applies if we’re going to ask, “But why do you accept X and not Y.” You can only accept an answer coming from the framework of the answer giver. If Gilbert doesn’t accept any sort of fall, take it from there with your objections above. If he does… then we’re left with something along the lines of my answer, which I believe to be in line with Catholic dogma.

      • Nathaniel

        Get back to me when using contraception is listed as a mental illness in the DSM.

        And to show just even more how bullshit your analogy is, it isn’t about weight loss. If it was, then people would stop being anorexic or bulimic when they reached a certain target weight. They don’t, because its a self perpetuating mental illness, not a normal change in diet.

        I really shouldn’t be surprised to find someone like you pathologize sex by equating it with a mental diesease that’s been known to kill people.

    • Qualifier: I am a former Catholic. My wife is still a Catholic. We use NFP and do not use any forms of contraception.
      Alex’s reply is mostly correct. NFP’s acceptability is two part: one part mindset, one part mechanics.

      The Catholic Church forbids intercourse with a “contraceptive mindset.” Thus, NFP is wrong if used specifically to prevent pregnancy, unless one has a grave reason. Leah’s written about this before, referencing Why doesn’t the Church just make a list. In any case, the Catholic Church doesn’t spell out what qualifies as a “grave moral reason.”

      The what I’ll call “clear conscience loophole” involves mechanics. Since sperm still enters the vagina uninhibited (yeah, I said it), Catholics get the boost of feeling “open to life.” When you chart, however, it’s going to take a f***ing miracle to get pregnant, say, 5+ days after “peak day” (day most likely to be the day of ovulation) or during menstruation. So, I’d caution elevating NFP to the status of being perpetually “open to life” simply because the sperm are uninhibited in their travels.

      As an aside, since other methods are not 100% effective, wouldn’t they qualify as being “open to life” as well? The only truly non-open-to-life form of intercourse is no intercourse, at least with current technology.

  • Also, I’d add one big caveat to effectiveness. Chemical/mechanical contraception is more easily separated from emotions/motivation (intuition speaking).
    — Decide to use it
    — Use it properly
    — Have sex

    NFP is much more linked to willpower and emotion. Want to be intimate with your spouse? Don’t want a kid? Get out the Scrabble board or go for a walk. While you are correct, Leah, that NFP requires a decision and classroom time… there’s no real classroom practice for dealing with what it feels like to get into bed next to the attractive person you’re married to and just do nothing.

    And, lastly, if we’re assuming that “Catholic”==”True Catholic,” then Catholics also have no path forward in terms of non-reproduction-risk sexual activity. It’s vaginal penetration or nothing. There’s probably no way to find this out, but I’d be interested in knowing if, say, a couple was out of condoms at the moment and chose a non-risky path of some other form of stimulation vs. risking pregnancy by proceeding with full out intercourse.

    Alright, enough sex talk for me today.