The use of standardised questionnaires with general health measures provides the opportunity to compare the health profiles of groups with different diagnoses, illness severities, or treatment regimens; to monitor transitions in health status over time ; to measure the burden of disease in populations with chronic and psychiatric diseases and in healthy populations; and to compare health outcomes across different health systems . The standardised Short Form Health Survey 36 (SF-36) is one of the most common instruments used in health research, both in population-based surveys and in studies to evaluate health policies . Its aim is to detect medically- and socially-relevant differences in health status and changes in health status over time using a small number of statistically-efficient dimensions. For this purpose, a multi-item scale was developed that employed multidimensional health concepts used in comprehensive health surveys, including measures of well-being and self-evaluation of health status [4–6]. The items in the questionnaire were selected from the set of 149 items of the Functioning and Well-Being Profile, which covered 40 health concepts used in the Medical Outcomes Study (MOS), and organised in a standard version, which is available since 1990 . The Short Form 36 (SF-36) consists of 36 questions: one of them measures health transitions over a one-year period and is not used in scale calculation, and the remaining questions are grouped into eight scales or domains. The eight scales can be aggregated into two independent summary measures: physical component summary (PCS) and mental component summary (MCS). Higher scores indicate better health.
The SF-36 was translated into various languages and used in several countries to assess the health perceptions of both the general population and people affected by disease [4, 7]. Even though its accuracy is 10% to 20% lower than that of longer questionnaires used in the MOS, its completion time of 5-10 minutes, versatility of use (self-completion, personal or telephone interview with persons aged over 14 years), and levels of reliability and validity above the recommended minimum standards make it an attractive tool for use in combination with other questionnaires in population surveys. Study results show that the SF-36 meets the criteria for data quality and scaling assumptions: the two main components used in the scales -- Physical (PCS) and Mental (MCS) -- explained 74% of the total variance. Experiences using the questionnaire and its reported shortcomings, such as cross-cultural non-equivalence, difficulties with some word meanings, floor and ceiling effects, poor performance of the two Role Function scales and standard layout, were used as a basis for implementing changes in the second version (v.2) of the SF-36, in use since 1996 . These changes included adjusting the layout horizontally, improving the wording of questions to make them less ambiguous, changing the response options of items related to Social and Emotional Functioning from binary to ordinal, eliminating one response option from the Vitality and Mental Health scales, and normalising scale values in order to improve comparability among different groups . The results of studies that used the SF-36 version 2 showed an improvement in accuracy, reliability and validity, without compromising the underlying structure of the conceptual model [6, 9].
In Brazil, the SF-36 was used in studies on the quality of life of patients with end stage renal disease undergoing intermittent haemodialysis , hypertensive patients , patients subjected to surgical repair of hip fracture , patients living with HIV/AIDS , and in a household survey of residents of the state of São Paulo . In these studies, the scores for SF-36 domains obtained in adult populations showed high reliability and good criterion validity compared to other instruments for assessing quality of life. In 2008, a survey on the social dimensions of inequality named Pesquisa Dimensões Sociais das Desigualdades (PDSD), coordinated by Instituto Universitário de Pesquisas do Rio de Janeiro (IUPERJ) with the participation of various teaching and research institutions in Brazil (UFMG, UFF, FIOCRUZ, UFRJ, PUC-RJ, UFBA), interviewed people around the country to assess the current situation of the Brazilian society with regard to education, health, and professional paths, with the objective of informing social policies. The Health module of the SSDI evaluated several aspects of health using the standard SF-36 (v.2), whose questions relate to the 4 weeks prior to the interview. Unlike previous applications in the country, which dealt with limited samples of individuals with specific health problems, the PDSD used the SF-36 on a probability sample of Brazilian households, thus estimating national scores to be used in future applications of this instrument. The aim of this paper is to assess whether the scales obtained from the SF-36 (v.2) questionnaire used in the PDSD project meet the minimum psychometric standards of data quality, scaling assumptions, reliability, and validity; reproduce the hypothesised mental and physical dimensions; and the relations between factors and scales predict their associations with external criteria for physical and mental health.