Would I have heard if the results were different?

The story about cycling and prostate cancer in the Herald (or the Daily Mail) is a good opportunity to look at some of the rules of thumb for deciding which stories to read or believe:

Firstly, would you have heard if the results were the other way around? Almost certainly not: prostate cancer wasn’t the main point of this study, and there wasn’t a previously-suspected relationship.

Second, for cancer specifically, is this mortality or diagnosis data? That is, are we seeing an increase in detection or in cancer? This is diagnosis data; so it could be just an increase in detection. The researchers were confident it wasn’t, but we must remember the immortal words of Mandy Rice-Davies

Thirdly, what sort of study is it? Obviously it can’t be experimental, but a good study design would be to ask people about cycling (or even better, measure cycling) and then see whether it’s the bike fanatics who develop cancer. This study was a self-selected survey of cyclists, getting self-reported data about past cycling and past diagnosis of prostate cancer. It’s a fairly extreme sample, too: half of them cycle more than 5.75 hours per week.

Fourth, how strong is the evidence of association, and what sort of sample size are we looking at? The association is just barely statistically significant (p=0.046 in one model, p=0.025 in a second), and there are only 36 prostate cancer cases in the sample.  It’s pretty borderline.  The estimated relative risk is huge, because it has to be given the sample size, but the uncertainty range is also huge. The confidence interval on the relative risk of 5 reported by the Herald goes from 1.5 to 18.

Fifth, what does previous research say? This is in the story

‘To the best of our knowledge, this is the first study to demonstrate an association between prostate cancer and cycling, so there are no studies hypothesizing a pathophysiological mechanism for such a link.’

Sixth, what do other experts think? We don’t know. The closest thing to an independent comment is this in the press release

“Physicians should discuss the potential risks and health benefits of cycling with their patients, and how it may impact their overall health,” says Ajay Nehra, MD, Editor-in-Chief of Journal of Men’s Health and Chair, Department of Urology, Director, Men’s Health, Rush University Medical Center, Chicago, IL.

He could have said that without reading the paper.

In summary, there’s borderline evidence from a weak study design for a sensational finding that isn’t supported by any prior evidence. This is fine as research, but it shouldn’t be in the headlines.

You can read the research paper here for the next month, and the journal press release here.