Item-level analyses
aches/pains, fatigue, abdominal pain, vomiting, pain, skin rash, muscle aches/pains
CHEST CONGESTION
Results from the cross-sectional study showed that all SIC inter-item correlations were positive in sign and correlations within each composite score were generally statistically significant and at least moderate in size (r ≥ 0.30).In the cross-sectional study, construct validity correlations between SIC items and PROMIS-29 scores followed the hypothesized patterns. As expected, the SIC fatigue score correlated positively with PROMIS-29 fatigue score (r = 0.64) and negatively with the physical function score (r = − 0.65); SIC physical weakness, muscle aches/pains, and joint aches/pains scores correlated moderately to strongly with PROMIS-29 physical function scores (r = − 0.67, r = − 0.52, and r = − 0.42, respectively); and the 5 SIC pain items correlated at least moderately with PROMIS-29 pain scores (all r ≥ 0.32).Respondents with mild or no symptoms on the PGIS had better item-level SIC scores compared with the moderate or severe PGIS subgroups. Most differences between PGIS subgroups were statistically significant, with the exception of the chest congestion, eye irritation, abdominal pain, and red/bruised feet items. Respondents with better PROMIS-29 physical function scores and better PROMIS-29 fatigue scores also had better SIC scores, with statistically significant differences observed except for the vomiting item (with PROMIS-29 physical function) and skin rash, uncontrollable shaking/shivering, and red/bruised feet (with PROMIS-29 fatigue). As expected, the largest mean difference between PROMIS-29 physical function subgroups was for the SIC physical weakness item and the largest difference between PROMIS-29 fatigue subgroups was for the SIC fatigue and physical weakness items. These item-level known-group analyses provided strong support for the discriminating ability of the SIC items.