[jdmitch]

Hurt's study enrolled only full-term babies so the possible effects of prematurity did not skew the results.

Doesn't this mean that their selection sample effectively excluded any babies that had a noticeable physical reaction (other than having cocaine in their system) to the effect of their mothers' cocaine use?

[jessriedel]

Yea, the outcome can be summarized as "No significant effect of cocaine exposure seen in subset of fetuses that are not born premature." But all this tells you is that premature birth is a necessary condition for bad effects (if they exist).

[acjohnson55]

Ideally, in an experimental design you want to control all but one variable. In real-life human studies, you can almost never do this exactly, but you try to approximate a true controlled experiment as much as possible. In this case, the independent variable is cocaine exposure, and full-term birth is a controlled variable.

[jlgreco]

Yeah, I don't really get that part either. Prematurity is something that is itself heavily studied; prematurity with crack seems like the logical thing to study.

[jasonlotito]

> prematurity with crack seems like the logical thing to study

It is a logical thing to study, and most likely is being studied, by someone else. After all, this study wasn't looking at premature babies. That doesn't prematurity with crack isn't a problem, only a different problem.

[jlgreco]

My issue is less with what the experiment studied and more with how it was summarized (particularly by the headline).

[sophacles]

Except... How do you know which effects come from crack + premature without subtracting out just premature and just crack?

[jlgreco]

Designing experiments in this area is not my expertise, but my inclination would be "study premature babies with and without crack" You could also simply study premature babies in general, and then study why crack seems to cause more premature babies.

I imagine somebody who is actually in this field would not have much trouble designing experiments to study premature "crack babies". Premature babies are something they chose not to study, not something that cannot be studied.

[jpttsn]

The provided justification ("so the possible effects of prematurity did not skew the results") is written in a somewhat self-gratifying way I think; simplifying the experimental design was surely another factor taken into account.

Excluding premature babies of course limits the study's relevance, though the results may be remarkable even if they only apply to full-term babies.

[samspot]

They wouldn't be able to tell which babies were premies because of the cocaine, and which were because of other factors.

A study of only premies would be interesting, but might yield the same result as this study.

[clavalle]

Highly unlikely. Prematurely born crack babies are often in very, very bad shape.

The rate of premature birth among all pregnant women is around 4.3% (that includes smokers and drug users so the rate amongst healthy non-smokers is even lower than that). Amongst crack cocaine users the number is between 17–27%. So the prematurity issue is pretty significant. You might even say dominant.

[a3n]

You can't study everything at once.