Characteristics of the Washington Group Short Set in Assessing Functional Difficulty Among People With Physical...

The Washington Group Short Set on Functioning (WG-SS), consisting of six questions on functional difficulties, is an international disability measure aiming to estimate disability prevalence and disability-related inequalities globally. In 2022, the WG-SS was introduced for the first time in Japan, necessitating a better understanding of the characteristics of the new measure compared with an existing classic disability indicator in Japan, i.e., possession of a physical disability certificate (PDC). Considering that the PDC is issued for individuals with mild to severe impairment, individuals with mild impairment might disproportionately be uncaptured by the WG-SS. For verifying this hypothesis, this study aimed to (1) examine the relationship between the severity of impairment and WG-SS judgment, and (2) describe percentages of individuals captured as "not having disability" based on the WG-SS stratified by severity of impairment among individuals who had the PDC of vision, hearing, or lower-limb impairments.

Secondary dataset of a representative Japanese individuals with disability (n = 14,079) was used for this cross-sectional study. From the dataset, data of individuals who had PDC of vision, hearing, or lower-limb impairments were extracted for analysis. All statistical analyses were conducted distinctively by the impairment group. After examining the relationship between severity of impairments and WG-SS judgement using univariate and multivariate analysis, percentages of individuals captured as "not having disability" based on a recommended cut point of the WG-SS, a response of "a lot of difficulty" or "cannot do at all" on at least one of the six questions, were calculated and described stratified by severity of impairment.

The number of eligible study participants for analysis was as follows: PDC holders for vision impairment (n = 189), hearing impairment (n = 240), and lower-limb impairment (n = 634). The univariate or multivariate analysis showed significant relationships between the severity of impairment (mild impairment) and the difficulty category of corresponding WG-SS individual questions (mild difficulty, i.e., "no difficulty" or "some difficulty") for each group. Among the PDC holders of vision impairments, the percentages of individuals captured as "not having disability" based on the WG-SS were 16.7% and 5.9% in the mild (n = 36) and severe (n = 153) impairment subgroups, respectively. Similarly, 39.0% and 20.2% in the mild (n = 146) and severe (n = 94) hearing impairment subgroups, respectively, and 54.0% and 28.9% in the mild (n = 430) and severe (n = 204) lower-limb impairment subgroups, respectively.

PDC holders with mild impairment were less likely to be captured by the WG-SS as "having disability" than those with severe impairment. These findings suggested a characteristic of the WG-SS that the disability statistics based on the WG-SS may underrepresent PDC holders with mild impairment. Stakeholders in Japan need to take into account this characteristic when they use or interpret the WG-SS.