Differential item functioning analysis on the Geriatric Depression Scale-15: an iterative hybrid ordinal logistic regression

The elderly population has extensively increased globally, so depression like a common problem in late life may convert to one of the economic, social, and health challenges of the 21st century. Due to the high cost of clinical diagnosis of depression, it is necessary to provide effective questionnaires like the 15-item Geriatric Depression Scale (GDS-15) for screening. But, the measurement invariance of GDS-15 is still unknown in the general population. In our study, 1473 participants of all Iran's ethnic groups were asked to answer GDS-15 and demographic factors such as human settle-ments, employment, disease, marital status, age, gender, homebound, financial status, and ethnicity. Then, the lordif package in R 3.1.3 was used to assess differential item functioning (DIF) items that behave unevenly across demographic factors. Our findings reveal that women, homebound patients, poorer, and non-Persian mother tongue score classic psychological symptoms higher than peoples of the same depression score in other groups. Since, psychologists have to remove or replace these items before using this questionnaire for screening geriatric depression.