Abstinence pledges don't reduce STDs

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WaPo reports on the not-too-surprising result that abstinence pledges don't have any significant effect on STD rates. Students were divided into 3 groups: non-pledgers, inconsistent pledgers (who changed their status or responses) and consistent pledgers. Here's a summary table of the results:

-Non-pledgersInconsistent PledgersConsistent Pledgersp
STD rate 6.9 6.4 4.6 .150
Sex before marriage 90 79 61 ?
Condom use (first sex) 59.7 54.9 54.6 .017
Oral sex only 2 5 13 <=.000

My take: On the one hand, this suggests that abstinence pledges aren't very useful in preventing STDs. On the other hand, while there's a statistically significant difference in condom usage between the three groups, it's only about a 10% difference, which really isn't that big a deal. There appears to be a real difference in the rate of premarital sex, but I wonder how much of that is an artifact. People who don't plan to have premarital sex are probably a lot more likely to take pledges in the first place, I would imagine.

It's particularly interesting to note the substantially higher rate (6 times) of "technical virgins" (those who have only had oral sex) in the pledging group. There are (at least) two possible explanations for this: (1) people who take the abstinence pledge really do want to have sex but somehow feel committed and so take advantage of the oral sex loophole. (2) people who think of oral sex and intercourse as different are more likely to take abstinence pledges. Note that when you add up the total "sexual contact" rates you get 92, 84, and 74, respectively. Nearly 3/4 of people who pledge not to have sex have some significant sexual conduct anyway. Somehow I suspect that's not exactly the result the abstinence advocates were looking for.

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I'm probably showing my ignorance here, (despite last-minute revision) but I read the results as indicating that there is only a 15% chance that STD rates are no lower among pledgers. The meaning of the result has been mangled (by the authors) from "not conclusively showing STD rates are lower" to "showing STD rates are no lower", when that's the thing the results definitely don't show.

In other words, I smell spin.

That's not quite what it means. Technically, if the rates of STDs between pledgers and non-pledgers were equal, if you did a whole bunch of random samples, 15% would should show a difference this large or larger. This is a fine, but important distinction. Consider the case of a coin you know to be fair which you flip 8 times in a row. Do that 1000 times, about 125 of those trials will produce 8 heads, but if you get 8 heads in a row, that doesn't mean that there's a 12.5% chance the coin isn't fair (since we've already stipulated that it is...)

Of course, in the real world we don't know whether the coin is fair. That's what we're trying to find out. But precisely because some tests will come up "positive" by random chance, we need to have some criterion after which we conclude that the result is negative. The conventional place where this line is drawn is p

Isn't it a little strong to describe this study as presenting the "result that abstinence pledges don't have any significant effect on STD rates"? After all, the non-pledgers in the study had STDs at a 50 percent higher rate than consistent pledgers. If that kind of difference is still not statistically significant, then it sounds like there's a serious problem of insufficient sample size. In that case, the most you could say is that the researchers in this study did not find any evidence of any effect--not that their study makes the absence of any effect significantly more likely.

Or are the researchers assuming a model in which any effect of consistent pledging also has to show up for inconsistent pledgers as well? (I can't see the full study.) Perhaps we should conclude, then, that inconsistent pledging turns out to be useless at reducing STD rates, but that consistent pledging may (or may not) actually help?

[Potentially confused comment by EKR attributed to lack of sleep and deleted]

I looked at 6 similar studies in detail last year for a class. They all appeared to measure the pressure on children to make pledges.

My favorite study looked at schools in the Boston area. At some of the schools, the parents were present with the children when they were asked to pledge. As you might expect, these schools showed a statistically significant increase in pledges but no real difference in pregnancy rates.

The study, for my money, shows that there is likely a lower rate of STDs among pledgers. Essentially, there is an 85% chance that the results are not due to random error. So if you were forced to bet on the question, you'd be stupid to bet that the rates were equal. Anyway several of you are right, you can never accept the null hypothesis (group 1 = group 2), you can only fail to reject it.

Why the study is underpowered to detect its main endpoint is a good question to ask. I would think you'd choose to have enough power (samples) to make a conclusion if the measured difference was as great as it was in this study! Usually one says "If the difference is 20% or greater, I want to know that conclusively" and then you get enough samples for that test. You're talking hundreds less kids with STDs in this study sample alone... nationally it would be hundreds of thousands per year.

I don't know where I stand on this issue because the reporting is so crappy on both sides. You certainly can't believe the religious right's reporting but the fact that the regular media outlets trumpet this type of result triumphantly (and with wrong conclusions as we've noted) while downplaying the fact that these studies do seem to show that pregnancy rates are lower is the type of bias that unfair and unbalanced media use to justify their existence. What a downward spiral we're in, and all because kids love sex. Thanks for the libido, God/natural selection.

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