Why doctors need to know Bayes theorem

In graduate school, I was the head TF for several general audience statistics courses, and my favorite subject was Bayes theorem because it implies that many "common sense" policies are, in fact, dangerous. Given a dreaded disease, drug use among ship captains or pilots, or anything else, it's so easy to say, "Just test everyone." But in fact, that's not good policy.

Social psychologist Gird Gigerenzer's new book covers some instances of asking doctors to give probabilities to their patients, and they do a horrible job. The question presents the information exactly as doctors are taught: prevalence, sensitivity, and specificity (false positives).

The probability that one of these women has breast cancer is 0.8 percent. If a woman has breast cancer, the probability is 90 percent that she will have a positive mammogram. If a woman does not have breast cancer, the probability is 7 percent that she will still have a positive mammogram. Imagine a woman who has a positive mammogram. What is the probability that she actually has breast cancer?

A prestigious doctor, department chief with 30 years of experience "was visibly nervous while trying to figure out what he would tell the woman. After mulling the numbers over, he finally estimated the woman’s probability of having breast cancer, given that she has a positive mammogram, to be 90 percent. Nervously, he added, ‘Oh, what nonsense. I can’t do this. You should test my daughter; she is studying medicine.’ He knew that his estimate was wrong, but he did not know how to reason better. Despite the fact that he had spent 10 minutes wringing his mind for an answer, he could not figure out how to draw a sound inference from the probabilities."

And he was typical: more than 90% of the doctors were wrong, mostly very wrong.

When the question was posed in terms that are easier for people to understand, nearly all of the doctors got the question right.

Sweden has had sex education since 1918

While doing a literature review for a paper about syphilis, I learned that Sweden has had what we would now call comprehensive sex education since 1918 ("Venereal Diseases and Sex Education", Report by a Swedish Government Committee, British Medical Journal, vol 1, no. 3204, May 27, 1922, p.842), as part of their "venereal disease" prevention law.

A 1943 discussion of sex education shows that current discussions are almost identical to past dicussions. The Archbishop of Canterbury noted that "if men and women would abstain from fornication the problem would be reduced greatly and become a purely medical matter. The bulk of the evil is primarily a moral problem." and he suggested that by distributing condoms to the troops "the implication is that many are expected to practice fornication" and thus it increases. One person cited one example in which moral suasion worked to prevent sex among soldiers --- the Black Sea Army in World War I --- and that the forces would respond if chastity were portrayed "in the right way".

On the other side, a medical officer from St. Pancras argued that the Church's teaching was unhelpful and that creating a taboo against premarital sex causes those who break the taboo to become rebels and outcasts. An alderman calls for sex education for those age 13 and up since otherwise they will learn about sex from "street talk", while a doctor calls for sex ed starting at age 11. Meanwhile, a minister in the Church said that he has discussed venereal diseases openly for years, and in a separate report, a psychiatrist noted that "The clergy were more broad-minded than schoolmasters."

Three months without federal abstinence-only funding

The US had 3 months without abstinence-only funding on the books before the health reform law reinstated it: $50 million a year for 5 years for the states, the exact same level of state-level abstinence funding as in the previous administration, even though fewer states are applying for the funds. Even Alaska under Sarah Palin "missed the deadline" for abstinence grants in the last round. (No Sarah Palin jokes. Missing the deadline just seems to be the politically expedient method of not participating in abstinence funding.)

The difference between current funding and the previous administration seems to be $154 million: community organizations are no longer given abstinence grants ($141 million in 2009) and the Adolescent Family Life abstinence program is no longer listed in the budget ($13 million in 2009). Not funding AFL is surprising, and I wonder if I've missed something since that program has been funded consistently since the early 1980s, but I don't see in the 2011 HHS budget; this program generally flies under the radar and is usually not mentioned in media reports about abstinence education, even though organizations like Advocates for Youth track it.

The decrease in abstinence-only education funding from $204 million to $50 million represents additional good news: the $50 million is going to states who are scrutinized more heavily. For instance, some states have laws requiring that all sex education be medically accurate. On the other hand, increasing numbers of states require contraception to be taught in sex education, which may mean that even if the states wanted to apply for the abstinence funds, their state law mandates a curriculum that is incompatible with the abstinence education requirements. The a-h definition of abstinence education in the Title V is so restrictive that the only abstinence-only program that has ever been shown effective (Jemmott and Jemmott's research) could not be funded under Title V.

Another difference between the pre-2009 and current HHS budgets is that abstinence was featured prominently in the budget in 2009 and before. Many pages of the pre-2009 budgets mentioned abstinence, and there was a whole subsection describing all the sources of abstinence funding and their histories. The 2011 budget doesn't have any text about abstinence and only includes abstinence in the itemized totals, mostly for past years, and has "--" where future years of abstinence funding are supposed to be listed. (The new 50 million a year isn't yet listed in the 2011 budget.)

While the country had only the 3 month period between the zero-ing out of abstinence education at the end of December 2009 and the passage of the health reform bill at the end of March 2010, there's certainly a big shift.

The really interesting question is how many states will apply for abstinence funds, since more states now have comprehensive sex education laws that are incompatible with applying for the funds, and whether it would be possible for the a-h definition to be changed. Or if the $50 million will go only to the states with strong abstinence-only constituencies such as Louisiana and Texas.

Sex Education of the Weird

A recent News of the Weird had two pieces of news related to sex education, but the weirdest was in this Thursday's New York Times.

First, abstinence-only sex education was the explanation for a teen's harassment of women:

Sheriff's deputies in Austin, Texas, arrested Anthony Gigliotti, 17, after complaints that the teen was annoying women by following them around in public and snapping photographs of their clothed body parts. Gigliotti told one deputy that he needed the photos because the sex education at his Lake Travis High School was inadequate. [KXAN-TV (Austin), 2-2-10]

On the other hand, condom education isn't necessarily safe either:

Clumsy: Teacher Karen Hollander filed a lawsuit in November against the New York City Department of Education after taking a fall on "slippery foreign substances," including condoms, on the floor at the High School of Art & Design. Since schools distribute condoms on campus, she said, the department is responsible when students open them and discard them during the lunch period, littering the floor. [New York Daily News, 11-21-09]

The weirdest of all, however, was in the New York Times. In response to a new comprehensive sex education law that requires contraception education to be included in any sex education course, the district attorney of a county in Wisconsin "warned that teachers face 'possible criminal liability' for teaching youths how to use contraceptives."

As the GAO found, some fraction of abstinence-only education just isn't true. While well-intentioned, this prosecutor's warning is no exception.

Do teens consider oral sex to be sex?

A recent study finds that only 20% of a convenience sample of teens consider oral sex to be sex. This finding has been documented anecdotally, such as in the classic scene from the 1994 movie _Clerks_, where the main character is described to hear the discrepancy in his girlfriend's number of sexual partners and number of oral sex partners. (3 vs. 37.) He had only been told about the former.

Of course there's the caveat that this convenience sample has no implications about US teens' norms in general since the sample wasn't random.

Public vs. private virginity pledges

The abstinence movement has been marked by grand gestures, and yet Melanie Bersamin found that teens who promise themselves to abstain until marriage are more likely to delay sex, while teens who take virginity pledges are not likely to delay sex. I gave a presentation about virginity pledges recently, so when I read the following, I thought of it:

The first tablets of the Ten Commandments, given with so much drama, were
destroyed. The second tablets, given privately and quietly, survived and became
the spiritual foundation of the people of Israel.

The Me'am Lo'ez points to the moral of this story: the really important and
lasting things in life are often done by individuals in privacy, through their
own exertions. Things done with much publicity may not be as permanent. (Marc Angel)

Perhaps the virginity pledge movement was a victim of its own success: the public nature of the pledges, the 1.5 billion in abstinence funding, and the attempts to seem trendy may have been the sound and fury signifying nothing of the first ten commandments.

Positive predictive values and gun control

The US has a number of workplace and school shootings, and each incident goes the same way. Media dig up events from the perpetrator's past (putting the perpetrator's name everywhere when the perpetrator's name really should forgotten, and the victims the ones who are remembered), including criminal record, complaints from co-workers, qualitative assessments of the shooter's mental health, and random anecdotes, and all of these events are assembled to answer the question of whether someone could have known that the perpetrator was so crazy.

While some of these incidents from a perpetrator's past are indeed crazy, they have poor positive predictive value: the most recent shooter apparently punched a woman over a booster seat at an IHOP. But such ironic and strange incidents happen all the time, which I know as an avid reader of News of the Weird. Attempting to predict who will go bezerk is like looking for a needle in a haystack: many people do minor strange, crazy things, and even do so multiple times, but most people who do minor strange things will not commit homicide. Even looking at more severe incidents, such as shooting a brother at age 20, may not necessarily predict future violence. Though allegedly sending a bomb to a dissertation committee member seems more likely to predict future violence, she was cleared of that charge.

Attempting to predict who will snap begs the question. We can't. Even if we had Big Brother comprehensive databases of every person's past, and were willing to disregard rules of evidence and dropped charges, we couldn't predict severe violence. The real issue is not which people are likely to snap, but rather why our gun control policy allows people such easy access to firearms that when they do snap they can do so much damage. Human psychology is fallible, but in countries without easy access to firearms, the damage comes in the form of broken objects, bruises and broken bones, and perhaps even a stabbing. Firearms create more damage, and damage that is most likely to be deadly.

. . .

Speaking of News of the Weird, here's one from last year about a Yale PhD and professor:

Love Can Mess You Up: Before Arthur David Horn met his future bride Lynette (a "metaphysical healer") in 1988, he was a tenured professor at Colorado State, with a Ph.D. in anthropology from Yale, teaching a mainstream course in human evolution. With Lynette's guidance (after a revelatory week with her in California's Trinity Mountains, searching for Bigfoot), Horn evolved, himself, resigning from Colorado State and seeking to remedy his inadequate Ivy League education. At a conference in Denver in September, Horn said he now realizes that humans come from an alien race of shape-shifting reptilians that continue to control civilization through the secretive leaders known as the Illuminati. Other panelists in Denver included enthusiasts describing their own experiences with various alien races. [Rocky Mountain Collegian, 9-28-09]