I tend to think that increasing access to contraception could somewhat reduce unintended pregnancy, and thus abortion and out-of-wedlock births. Unfortunately, a new study making claims to that effect, in which researchers tracked teens who'd been provided birth control free of charge through a program called CHOICE, has a serious problem.

Take a look at this chart the researchers provide, particularly the legend at the top:

Notice anything odd? Such as that the control group isn't actually a control group?

Normally, when a scientist wants to know whether something works and has the option of recruiting people and trying it out, he puts together a big group and then randomly assigns some to receive the treatment. Then he compares the folks who got the treatment with those who did not. This way he can be reasonably sure that the only difference between the two groups is whether they received the treatment.

It's easy enough to do this with contraception: Recruit women interested in receiving it for free, give it to just some of them, and then check to see what difference it made. Did the others buy contraception on their own? Were they more likely to get pregnant? But instead, the researchers simply compared teens who got free birth control through CHOICE with nationwide statistics. (The chart has the rates for all teens in the U.S., but see the full paper for some other comparisons, such as breakdowns by sexual activity, etc.)

This isn't nitpicking. More or less by definition, those who participated in the study were motivated enough to invest the time needed to obtain birth control (even if there was no financial cost). At the same time, the researchers' data (see Table 1) show that the participants were more likely than the general population to have numerous risk factors for unwanted pregnancy, such as low socioeconomic status. So the real difference could be lower or higher than the chart above suggests.

Again, I have no doubt that a difference exists -- basic economics would suggest that giving birth control away for free will increase the use of birth control (and especially expensive but effective forms of it like IUDs). But this study doesn't help us to quantify it.

Robert VerBruggen is editor of RealClearPolicy. Twitter: @RAVerBruggen