Measurement invariance of the SF-12 across European-American, Latina, and African-American postpartum women

The purpose of this study was to determine whether a postpartum-specific version of the SF-12 was invariant across three ethnic
groups. Specifically, we examined the presence of differential item functioning (DIF) among European-American, Latina, and
African-American mothers. DIF refers to differential endorsement of item responses that are not due to the construct being
measured. DIF can result in biased group comparisons.

We analyzed cross-sectional data of postpartum women (n = 655) who delivered at an urban hospital in the northeast region of the USA. Multiple indicators multiple causes (MIMIC)
model was used to examine differential item functioning.

The analyses revealed the presence of DIF for three items: Item 1 “self-assessed general health,” item 8 “bodily pain,” and
item 9 “calm and peaceful.” Only two DIF effects were meaningful based on odds ratios and on the percentage of the total effect
accounted for by the DIF effect. Specifically, African-American women differentially endorsed item 8 “bodily pain” when compared
to European-American women (OR = 2.11, CI₉₅ = 1.20, 3.71) and Latinas were more likely to endorse item 9 “calm and peaceful” when compared to European-American women
(OR = 2.62, CI₉₅ = 1.64, 4.17).

The results of this study indicate that the SF-12 is to a great degree an invariant measure for the assessment of HRQoL among
postpartum ethnically diverse women. More research is needed to examine other aspects of invariance (e.g., configural and
metric) and longitudinal invariance in ethnically diverse samples. To better understand ethnic differences in health, future
studies need to examine the factors that may underlie DIF effects in quality of life.