Glottal Function Index: Validity and Reliability of the Persian Language Version in Patients With Voice Disorders

Objective. To translate and cross-culturally adapt the Glottal Function Index into the Persian language (GFIp) and validate it in patients with voice disorders. Study design. A cross-sectional and prospective validation design was adopted. Method. The GFI was translated and culturally adapted into Persian language according to the methodology of standard forward-backward translations to obtain semantic, idiomatic, and conceptual equivalence. One hundred patients with voice disorders (53 men; mean age: 41.4 +/- 13.6 years) and 40 healthy volunteers (21 women; mean age: 36.7 +/- 10.0 years) completed the GFIp. Patients with voice disorders also completed the Persian Voice Handicap Index (VHIp) to assess the construct validity. Results. There were no floor and ceiling effects. Evidence for construct validity was found with a significant very good correlation between the GFIp and the VHIp total scores (r = 0.70; P < 0.001). Differences of GFIp scores between the patients and healthy participants were statistically significant (P < 0.001) confirming discriminant validity. The internal consistency reliability was acceptable for GFIp (Cronbach's a = 0.74). Absolute reliability measures of Standard Error of Measurement and the Smallest Detectable Change for GFIp were 2.5 (confidence interval 95% = +/- 4.9) and 6.93, respectively. Factor analysis revealed the GFIp as a single factor instrument. Conclusion. The GFIp is a valid and reliable self-administered instrument for use in Persian-speaking patients with voice disorders.