Does the single-item self-rated health measure the same thing across different wordings? Construct validity study

PURPOSE: The self-rated health (SRH) item is frequently used in health surveys but variations of its form (wording, response options) may hinder comparisons between versions over time or across surveys. The objectives were to determine (a) whether three SRH forms are equivalent, (b) the form with the best construct validity and (c) the best coding scheme to maximize equivalence across forms. METHODS: We used data from 58,023 respondents of the Swiss Health Survey. Three SRH forms were used. Response options varied across forms and we explored four coding schemes (two considering SRH as continuous, two as dichotomous). Construct validity of the SRH was assessed using 34 health predictors to estimate the explained variance. RESULTS: Distributions of response options were similar across SRH forms, except for the "good" and "very good" options ("good" in form 1: 58.6%, form 2: 65.0% and form 3: 44.1%). Explained variances differed across SRH forms, with form 3 providing the best overall explained variance, regardless of coding schemes. The linear coding scheme maximised the equivalence across SRH forms. CONCLUSION: The three SRH forms were not equivalent in terms of construct validity. Studies examining the evolution of SRH over time with surveys using different forms should use the linear coding scheme to maximise equivalence between SRH forms.