Application of Total Deviation Index in Aging Studies

Aims: Obtaining measurement errors in aging samples studies is of high importance mainly because of the study itself as well as the attention and consideration that are required in recording such data. This study aims to introduce a simple formula to obtain and interpret the Total Deviation Index (TDI) and encourage researchers to use it as a suitable tool in defining reliability in psychometrics studies. Methods and materials: In a cross sectional study in the year 2019, 105 aging individuals were selected from city of Tehran. The SF-36 questionnaire was completed for them within two weeks. The ICC value and TDI were obtained for each subscale and compared together. Results: The results show that the lowest TDI value belongs to physical functioning subscale, while mental health has the highest. To be more accurate, the maximum measurement error in physical functioning and mental health, with 95% likelihood, is 22 and 34 (of 100 units), respectively. Conclusion: Considering the easy interpretation of Total Deviation Index, also due to ICC index not being capable of identifying how much of the subscales variations come from measurement errors, it is suggested that researchers report and obtain the Total Deviation Index in their reliability studies.