Test-Retest Reliability of the Self-Reported Impairments in Persons With Late Effects of Polio (SIPP) Rating Scale

Background: A new 13-item rating scale, the Self-Reported Impairments in Persons with Late Effects of Polio (SIPP), has been developed. The SIPP has been analyzed using the Rasch method and has shown good construct validity and internal consistency. To establish its clinical utility, further evaluation of its psychometric properties is needed.

Objective: To evaluate the test-retest reliability of the SIPP and to define limits for the smallest change that indicates a real change, both for a group of persons and a single individual.

Design: A postal survey.

Setting: University Hospital.

Participants: Fifty-one persons (31 men and 20 women; mean age, 72 years) with clinically verified late effects of polio.

Intervention: Not applicable.

Main outcome measurements: The participants completed the SIPP twice, 2 weeks apart. The response frequencies at test occasion 1 (T1) and test occasion 2 (T2) were calculated. Test-retest reliability was analyzed using the percentage agreement of each item, the intraclass correlation coefficient, and the mean difference between the test occasions (đ), together with the 95% confidence intervals for đ, the standard error of measurement, the smallest real difference, and a Bland-Altman plot.

Results: The percentage agreement (ie, the same scoring at both test occasions) was >70% for 10 of 13 items. The mean score (standard deviation) was 27.9 (5.7) points at T1 and 28.2 (6.0) points at T2, with no systematic difference between the test occasions. The intraclass correlation coefficient was 0.88, the standard error of measurement (the smallest change for a group of persons) was 2.0 points, and the smallest real difference (the smallest change for a single individual) was 5.6 points, respectively.

Conclusion: The SIPP is a reliable rating scale in persons with late effects of polio and can be used to evaluate effects of rehabilitation interventions and changes of perceived impairments over time both for a group of persons and for a single individual.