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Thursday, 30 April 2009

Absolute VS Relative Risk Reduction


I've noticed a definite trend with pro circumcisers and statistics. Now as we all know, you can twist statistics to mean pretty much anything you want, and one way to do it is by using absolute and relative risks when it suits you. So, with circumcision, use absolute risk when you're talking about complications of circumcision (and take the smallest possible figure) Then when talking about the potential benefits, use relative risks. Because, when you're talking about a very, very small number, using relative risk makes your gain look MUCH bigger.

Take HIV, as the press is really pushing the alleged link with circumcision at the moment after the 3 Africa studies.  Apparently, the studies showed a 60% reduction in HIV acquisition in the 2 years after voluntary, adult circumcision. Note the latter part, as that is all it does show, since the studies were halted early we do not yet know if circumcision has an effect beyond that, and it is quite possible that the effect becomes lesser as time goes on (Observation of country prevalence of HIV certainly does not bear out that circumcision affects HIV, see the CIA Factbook for that. Note that the United States is right up there as one of the most HIV ridden countries despite around 80% of adult males currently being circumcised there)

The important point is though in terms of absolute vs relative risk reduction is what is the actual risk of catching HIV? Now, I'll admit that I always thought if you had sex with an HIV positive person you had a nigh on 100% chance of catching it. So, I'd forgive people for taking these figures as - 100% chance with unprotected sex, 40% chance with circumcision with the 60% risk reduction, and around 1% with a condom. Schoen touches on the actual risk factor mentioning a figure of 1 in 300, but does not elaborate on what this actually means.

First off where does this figure actually come from? Is he referring to the chance of contracting HIV when having sex with an HIV positive person, or the risk overall of a heterosexual male contracting the disease? It isn't at all clear, and it really seems to me that he simply cherry picked the best possible stat to meet his needs.   A blanket "1 in 300" means very little, because risk of transmission varies by country due to HIV prevalence, and of course viral load in the individual can vary. Some conditions like genital ulcers can make the risk of transmission higher. 

I've heard a lot of parents who circumcise their children state that they want their child to have an extra layer of protection for the "just in case" scenario if a condom breaks or whatever.  Sounds very reasonable, until you look at the actual probability of this occurring. 

This excellent post by Fellow Traveller at Mothering.com spells it out rather well. Quote:

"We'll start the estimate that the chance of infection is 0.06% a bit higher than published in the Lancet article. That means a male having unprotected sex with an HIV positive woman has about a bit more than 1 in 1800 chance of being infected. Base line risk intact men vs circumcised men 1 heterosexual contact with an HIV+ partner.

[1 - 0.0006]^1 99.94% ~= 0.06%
[1 - 0.0006 * 0.5]^1 ~= 99.97% ~= 0.03%

But the HIV distribution in the US population is about 5 in 1000 or 1/200 so, in general, there is only a 1 in 200 chance that one will encounter someone who is HIV positive. Actually, the risk is much lower but we'll discuss that in a bit. Given that the HIV prevalence in the general population is about 1/200, a closer estimate of the risk of becoming HIV infected after the 1 random heterosexual encounters is more like:

The chance of event A (encountering an HIV positive individual in the general population) * the chance of event B the likely hood of getting infected during that encounter.

1/200 * 0.0006 = 0.000003 --- 1 - 0.000003 = 99.9997% = 0.0003%
1/200 * 0.0003 = 0.000006 --- 1 - 0.0000015 = 99.99985% = 0.00015%

Now, the number of sexual encounters is important too. For 1,000 encounters, the difference is 1.5 hundredths of a percent. That's is what circumcision bought you, big deal. Over the course of 1,000 random encounters, on average, an intact guy in the US has 1.5 hundredths of a percent larger chance of becoming HIV positive. Circumcised guys, party on!"

So - parents are being given the information that circumcision will reduce their child's risk of HIV acquisition by 60%, but fail to mention that the actual risk of their child a) coming into contact with an HIV positive partner b) condom failing and c) contracting the virus is a tiny, tiny, fraction.  A 60% reduction of a miniscule base is still miniscule. 

It's also worth looking at the British response to this article here:

"Dr Colm O'Mahony, a sexual health expert from the Countess of Chester Foundation Trust Hospital in Chester, said the US had an "obsession" with circumcision being the answer to controlling sexually transmitted infections. 

He said: "Sure, a dry skinned penis is a bit less likely to contract HIV, herpes and possibly genital warts but it will get infected eventually."

Sunday, 26 April 2009

Twitter and Breastfeeding..

Recently I've joined Twitter - well the point was for me to combat certain pro-circ folks posting there as one particular individual was using it to "tweet" new parents trying to convince them to cut their kids. (Creepy.) Anyway, a few days after and it's pretty addictive. Meeting new mama friends and I'm generally wittering on about rubbish to anyone who seems vaguely interested.

The AP community seems pretty strong. Lots of like minded people. Life is good.

And then - someone posts a link to that infamous "Breastfeeding at 8" video. As if this hasn't been posted a million times over on the internet and every single time it comes up, a huge flame war erupts, people get nasty, others get upset, etcetcetc. Been there, seen it a million times. I should never have got involved. But, I tweeted that as far as I am concerned, people should just let others live their lives, as long as they're not hurting their kids, who really cares.

This is where a certain lady who had followed me due to a few tweets on #flylady got really upset and started sending me about 10 tweets per minute (or so it seemed) on how morally corrupt I am, how I don't teach my children right from wrong, and how this makes me a terrible parent. This particular lady apparently weaned her baby as soon as he could walk, because apparently it's morally wrong to breastfeed a child after this point. At one point she brought my obvious stance on circumcision into the mix.

Now, get this straight. I am openly against infant circumcision. I post links about it, blog posts I find interesting (Much the same as I do breastfeeding, vax or other stuff) I will combat blatant misinformation I see posted about. If someone posts something like "what do you think about circumcision" I'll give my input. But I don't tweet people and insult them/their decisions and attack their moral fibre. Circumcision is also different from breastfeeding in that there are no damaging effects from breastfeeding. It has no risks attached. The benefits from breastfeeding are diverse and well documented, while with circumcision there are pretty much equal information on both sides of the argument (which is why, to me, it is more of an ethical issue. People should be given the chance to say whether they have healthy body parts removed, or not. The AAP states that the medical benefits of circumcision do not outweigh the risks therefore it is a cosmetic/cultural issue.)

As for breastfeeding - it is recommended, universally, for the first two years and beyond, as per the WHO and various other organisations. We've seen a few opinion pieces slating it lately (eg Hanna Rosin) But this does not change the fact that breastfeeding ultimately promotes optimal child health. Extended, or to use the term I prefer, "full term" breastfeeding, is attacked by some prudish types who see it as somehow incestuous and psychologically damaging. This is ridiculous. There have never been any studies on it proving psychological damage due to breastfeeding. However, the list of studies proving benefits to continuing the breastfeeding relationship is extensive.

I have noticed that people who did not breastfeed, or who breastfed for a short time, tend to be very vocal about others not judging them for their choices, yet will happily dish it  out when it comes to us full term breastfeeding mothers.  I don't go around randomly saying to formula feeding mothers that they are bad parents and made bad decisions.  I don't personally do formula, but I get that other people have the right to decide for themselves.  Lactivism is not about personally attacking others - it's about helping breastfeeding mothers by getting better support, campaigning for better breastfeeding in public laws, etc.

The final straw really in this particular discourse was that she commented that breast is best "but only until children can get nutrition from other foods". This is one thing that bothers me - people really think that if you're breastfeeding a child they're not eating anything else? My toddler is allergic to dairy, so he drinks my milk like he might have cows milk, an occasional and comforting drink with the bonus that it's attached to mama and he gets a cuddle at the same time.

And then, there's the people that say "After XX years, why not pump and give it to them IN A CUP!" I am sure the people saying this have never, ever pumped. Pumping sucks. You have to pump regularly to ensure you will get a decent amount of milk each time. It's time consuming, and soul destroying. I did everything I could to try and ensure I didn't have to pump with my second son - including negotiating work hours that would enable me to nurse before and after work without having to have a pumping break in between.  Why would I want to give up the cuddling closeness that comes with nursing (it's a fabulous way to connect with a toddler who is generally too busy for hugs otherwise) and replace that with attaching myself to a plastic device multiple times a day, just so to not offend other people's delicate sensibilities?