Janessa Graves, the author of the bikeshare helmet study showing that the percentage of injuries that were head injuries for cyclists went up in bikeshare cities, responded to criticism of the study. That criticism focused on the fact that the study's raw data showed that total head injuries and total injuries both went down in bike share cities, even though the rate of head injuries went down less than other injuries.
The study’s lead researcher, Janessa Graves of Washington State University’s College of Nursing, said in an email that these numbers don’t tell the whole story, which is why the researchers focused on proportion.
“Evaluating crude numbers alone, without considering the underlying population or denominator (e.g. number of riders in each city), is not entirely appropriate, even when we assume ridership increased,” she writes. “We did not have those numbers for this study, so could not evaluate the NUMBER or RATE [emphasis hers] of injuries. That is why we looked at proportions and risk.”
Graves adds that because her team did not know whether ridership increased, decreased or stayed the same in cities with bike-shares, they were reluctant to extrapolate. The total number of injuries may have gone down, but what of the total biker population?
Some critics of the study, she states “assume that the number of cyclists increased in bike-share cities and likely stayed the same in non-bike-share cities, however, we do not know this for certain. That is why we could not look at this outcome.”
Rachel Dovey concludes that
Shares would be wise to implement policies based on the higher proportion of brain injuries reported.
I'm not sure that is true, because looking at only the percentage without considering the denominator is just as bad (if not worse) as looking at the numerator without considering the denominator.
Shares should be interested in ways to improve safety. I'm just not sure what the most cost-effective way is to do that, and I don't think this study gives any direction on that.