Here we go again, says health journalism watchdog Gary Schwitzer of Health News Review, citing headlines that blare claims like “Coffee may reduce stroke risk!”

These media headlines introduced findings from a new study in which women who drank more than a cup of coffee a day had a 22% to 25% lower risk of stroke than those who drank less, according to Swedish researchers.

Their research was published in Stroke: Journal of the American Heart Association. Stroke is the third leading cause of death in North America, behind heart disease and cancer.

But Gary Schwitzer explains that although this was a big study (over 34,000 women ages 49-83, followed for an average of 10 years), it was only an observational study that can’t prove cause and effect.

“Not a trial. Not an experiment. And, as we say so many times on Health News Review, you could almost join along with the chorus – observational studies have inherent limitations that should always be mentioned in news stories.

“They can’t prove cause and effect. They can show a strong statistical association, but they can’t prove cause and effect. So you can’t prove benefit or risk reduction. And stories should say that.”

Some of the study’s limitations, for example, are appropriately outlined in a HealthDay news story:

“The problem with this type of study is that there are too many factors unaccounted for and association does not prove causality, said Dr. Larry B. Goldstein, director of the Duke Stroke Center at Duke University Medical Center.

“Subjects were asked about their past coffee consumption in a questionnaire and then followed over time. There is no way to know if they changed their behavior,” Goldstein said.

“And, he noted, there was no control for medication use or other potential but unmeasured factors.”

Schwitzer also slams the way that prominent media covered this “news” (including CBS News, ABC News, USA Today, and WebMD):

“WebMD was just plain inaccurate when it stated: ‘One or More Cups of Coffee a Day Reduces Stroke Risk in Women.’ The study did not prove that.”