The Impact of Differential Item Functioning on the Warwick-Edinburgh Mental Well-Being Scale.

Establishing the internal validity of psychometric instruments is an important research priority, and is especially vital for instruments that are used to collect data to guide public policy decisions. The Warwick-Edinburgh Mental Well-Being Scale (WEMWBS) is a well-established and widely-used instrument for assessing individual differences in well-being. The current analyses were motivated by concerns that metal wellbeing items that refer to interpersonal relationships (Items 9 and 12) may operate differently for those in a relationship compared to those not in a relationship. To assess this, the present study used item characteristic curves (ICC) and ANOVA of residuals to scrutinize the differential item functioning (DIF) of the 14 WEMWBS items for participant relationship status (n with partner = 261, n without partner = 210). Items 5, 9, and 12 showed evidence of DIF which impacted group mean differences. Item 5 ("energy to spare") was unexpected, however plausible explanation is discussed. For participants at the same level of mental wellbeing, those in a relationship scored higher on items 9 and 12 than those not in a relationship. This suggests these items are sensitive to non-wellbeing related variance associated with relationship status. Implications and future research directions are discussed.