The reliability of a questionnaire for repeating tests allows us to measure the extent to which a questionnaire provides the same values for repeated measurements, provided that the health of participants remains stable. It is measured using the Intraclassical Correlation Coefficient (CCI) in a mixed 2-way model with absolute agreement and the corresponding 95% confidence interval. A questionnaire is considered reliable if the ICC values obtained are greater than 0.70 . In this study, a sample of 278 Sarcopean subjects from 8 different countries and 9 different language groups established values for standard measurement error and smallest verifiable modification of the SarQoL questionnaire. The measurement error inherent in the questionnaire was 2.65 points and the minimum change had to be based on a 7.35-point change in the overall quality of a patient`s life. Systematic bias has been studied in more detail by the Bland-Altman method and has shown that there is no systematic distortion for almost all areas (with Domain 1 as an exception) and the total sarQoL questionnaire score. The average difference score () was calculated by calculating the average value of differences between the test and recidivism values for all subjects [Mean (test score-retest score)). The 95% limits of the agreement were calculated by the formula [± (1.96 SD difference)] [18,21]. The Bland-Altman plots were created in THE SPSS according to the instructions of the IBM Tech note number 19420 . The Sarcopenia Quality of Life (SarQoL) questionnaire, a patient-specific outcome measure, assesses quality of life with 55 articles. It produces 7 domain scores and 1 quality of life score, all between 0 and 100 points. This study aims to contribute to the interpretation of SarQoL scores by calculating standard measurement error (SEM) and smallest detectable variation (DDC) in a sample of subjects from 9 studies.
“… the standard measurement error deviation associated with test results for a particular group of test participants, AERA, APA – NCME (1985). Standard Measurement Error (SEm) is a measure of the amount of measured results distributed around a “true” value. The SEm is particularly useful for a test taker because it applies to a single score and uses the same units as the test. A total of 278 sarcopical subjects aged 77.67 ± 7.64 years and 61.5% of women were included. The SEM for the total value of SarQoL for each study was between 0.18 and 4.20 points and was 2.65 points if all subjects were analyzed jointly. The SDC for the overall score ranged from 0.49 to 11.65 points for each study and was 7.35 points for all subjects. The Bland-Altman plots did not reveal any systematic errors in the questionnaire. The SDCind for the total points of the SarQoL questionnaire in the full sample is 7.35 points. This means that an individual`s overall quality of life score of at least 7.35 points (on a scale of 0 to 100) should change before the observed change can be considered a real change in a subject`s quality of life and perhaps not as the result of a measurement error.
The SDCind for the 7 domains of the SarQoL questionnaire ranges from a minimum value of 10.27 points for Domain 4 to a maximum value of 25.56 points for Domain 6. The SDC group for the total points on the entire sample is 0.44 points. SDC values for the full example are available in Table 3. SDC values for the various studies provided are available in Table 4.