Measurement properties and normative data for the Norwegian SF-36: results from a general population survey

Garratt and Stavem Health and Quality of Life Outcomes (2017) 15:51 DOI 10.1186/s12955-017-0625-9 RESEARCH Open Access Measurement properties and normative data for the Norwegian SF-36: results from a general population survey AM. Garratt1* and K. Stavem2,3,4 Abstract Background: The interpretation of the SF-36 in Norwegian populations largely uses normative data from 1996. This study presents data for the general population from 2002–2003 which has been used for comparative purposes but has not been assessed for measurement properties. Methods: As part of the Norwegian Level of Living Survey 2002–2003, a postal survey was conducted comprising 9,164 members of the general population aged 16 years and over representative for Norway who received the Norwegian SF-36 version 1.2. The SF-36 was assessed against widely applied criteria including data completeness and assumptions relating to the construction and scoring of multi-item scales. Normative data are given for the eight SF-36 scales and the two summary scales (PCS, MCS) for eight age groups and gender. Results: There were 5,396 (58.9%) respondents. Item levels of missing data ranged from 0.6 to 3.0% with scale scores computable for 97.5 to 99.8% of respondents. All item-total correlations were above 0.4 and were of a similar level with the exceptions of the easiest and most difficult physical function items and two general health items. Cronbach’s alpha exceeded 0.8 for all scales. Under 5% of respondents scored at the floor for five scales. Role-physical had the highest floor effect (14.6%) and together with role-emotional had the highest ceiling effects (66.3-76.8%). With three exceptions for the eight age groups, females had lower scores than males across the eight health scales. The two youngest age groups (<30 years) had the highest scores for physical aspects of health; physical function, role-physical, bodily pain and general health. The age groups 40–49 and 60–69 years had the highest scores for role-emotional and mental health respectively. Conclusions: This SF-36 data meet necessary criteria for applications of normative data. The data is more recent, has more respondents including older people than the original Norwegian normative data from 1996, and can help the interpretation of SF-36 scores in applications that include clinical and health services research. Background The Short Form 36 (SF-36) Health Survey is the most evaluated health status instrument and the most reported within randomized controlled trials [1, 2]. The instrument has been translated into many languages and the results of these studies are published in peer-reviewed journals [3]. SF-36 Version 1 [4] and the RAND-36 [5] include the same items and continue to be widely used, including in the great majority of Norwegian studies that include this instrument. The SF-36 is available in self- or interview* Correspondence: 1 Knowledge Centre for the Health Services, Norwegian Institute of Public Health, PO Box 4404, Nydalen N-0403, Oslo, Norway Full list of author information is available at the end of the article administered formats and standard (four weeks) and acute (one week) recall periods. The SF-36 was developed as part of the Medical Outcomes Study (MOS), a key objective of which was to develop more practical tools for monitoring the outcomes of medical care [4, 6, 7]. The instrument includes 36 items or questions that assess functional health and well-being from the perspective of the patient. The items contribute to eight health domains of physical functioning, role limitations due to physical problems, bodily pain, general health, vitality, social functioning, role limitations due to emotional problems and mental health. The eight domains all contribute to physical component summary (PCS) and mental component summary (MCS) scores, © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Garratt and Stavem Health and Quality of Life Outcomes (2017) 15:51 Page 2 of 10 with their relative weights based on the results of factor analysis [8]. Short-forms include the SF-12 [9] and SF-8 [10] which give summary scores along with single item scores for each domain in the case of the latter. Normative data derived from surveys of representative samples of the general population aid the interpretation of the SF-36 scale and summary scores [11]. Normative data has been available following early evaluations of the instrument, for example as part of the International Quality of Life Assessment (IQOLA) Project [3, 12]. Much of this data was collected in the 1990s following forward backward translations and testing for crosscultural equivalence [3, 13, 14]. These normative data continue to be used [15–17] but more recent data is available for countries that were not included in the IQOLA Project [18–20]. The Norwegian SF-36 version 1.1 was forward backwards translated according to the IQOLA procedures and evaluated in patients with rheumatoid arthritis recruited from a patient register for Oslo [21]. Problems with missing data and suboptimal psychometric characteristics led to slight revisions to five items in version 1.2 [12], the one commonly used in Norway. This version was evaluated in a nationally representative sample of the Norwegian general population in the spring of 1996 and was used to derive the Norwegian norms [12]. The data is over 20 years old and may no longer be representative of the general population due to changes in both the composition of the general population and how individuals respond to such questions. The present study presents more recent normative data for the Norwegian SF-36 v1.2 [22]. This data has been used to help the interpretation of SF-36 scores in Norwegian studies since 2013 [23–25]. Compared to the original Norwegian norms [12], there are a larger number of respondents including older people, which further contributes to the appropriateness of this new normative data. However, the measurement properties of this normative data have not been reported. Norms are also given for the SF-36 summary scales, which were developed later and hence were not included in the original normative data. The study also presents norms for the two scales that have a different scoring algorithm according to the RAND scoring together with alternative scoring for the summary scales [26–28]. The present study follows the IQOLA project and existing studies that have evaluated th (...truncated)