Strengths and Limitations
Strengths
of this study include the use of a double-blind cross-over design
where neither the study participants nor the study staff (apart from
the lead investigator responsible for installing filters in the vehicles)
were aware of which vehicle had the cabin air filter in place on a
given day. As a cross-over study, participants acted as their own
control in the two filter conditions, reducing variability and confounding
from time in-varying factors. As with all interventional studies in
humans, the sample size was limited by what was feasible, reducing
the power of the study, although this was offset by the statistically
efficient cross-over design. We did not adjust for multiple comparisons
in our analysis but focused on the size and direction of the effect,
along with the 95% confidence intervals. This may have increased the
type I error rate.