Item characteristics
nasal obstruction, throat, pain
The individual symptom VAS scores were clustered at the high end of the scale at baseline, owing to the severity of symptoms present in the population. VAS responses were distributed across the full response scale at Weeks 20, 24 and 52. Ceiling effects were present at all time points for the loss of smell VAS, and a floor effect was observed for the facial pain or pressure VAS at Week 52. While no other VAS demonstrated substantial floor or ceiling effects, a higher proportion of responses were clustered at the upper versus lower end of the scale for all VAS scores at baseline (as was expected owing to the eligibility criteria), but not during the treatment period (Additional file All correlations between single VAS items were acceptable (> 0.3), with correlations > 0.9 observed only between two pairs of VAS: nasal obstruction VAS and nasal discharge VAS (r = 0.929), and nasal discharge VAS and mucus in throat VAS (r = 0.900). While a correlation of > 0.9 may show proof of redundancy, all items were included in the final model, as these item pairs include symptoms highly important to patients [