c Index of person separation, a Rasch based reliability statistic analogous to Cronbach's alpha/KR-20 [22, 35]. Indicates the degree to which people can be separated into discrete groups. Values of 0.7 and 0.8 are the minimum required to discern two and there groups, respectively .
d Items 4, 6, 7, 8, 11, 17 and 18 split by diagnosis.
NHPD, Nottingham Health Profile index of Distress; DIF, differential item functioning; SD, standard deviation; df, degrees of freedom.