Measuring QoL

SF-12 (Ware, 1996) (See appendix two)

Participants completed the SF-12 (Ware 1996); a subjective QoL self-report measure. The SF-12 is a shortened version of the SF-36 (Ware & Shelbourne, 1992) and generates two summary scores of physical health (PH) and Mental health (MH). Eight subscales are also derived: General Health, Physical Functioning, Role Physical, Role Emotional, Bodily Pain, Vitality, Mental Health, Social Functioning.

Awad (2000) suggests six points should be adhered to when measuring QoL in people with psychosis. These six points are outlined below with a rationale provided for the choice of measure in this study.

1. QoL is a multidimensional construct and this has to be reflected in its measurement. As can be seen from the description above the SF-12 provided a

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number of subscales assessing multiple components, representing the multidimensional nature of QoL.

2. The scale has to be appropriate to the population under study, the clinical condition and the phase of illness. It’s psychometrics have to be known and documented. Although the SF-12 scale was not designed specifically for a population with psychosis, both this scale and the longer SF-36 have been frequently and accurately used in this population. A global measure of QoL was chosen over a specific one for people with psychosis to enable the norms to be compared to a variety of populations. Utilising the SF-12 counters some of the criticisms levelled at subjective self-report measures, such as; the measures are often limited in terms of psychometric properties (Wilkinson et al, 2000). Both the SF-12 and SF-36 have been found to have high validity and reliability when implemented in this population (Pukrop et al, 2003; Vojta et al, 2001; Salyers et al, 2000; Russo et al, 1998; Tunis et al, 1999; Leidy et al, 1998). The SF-36 was found to have good test-retest reliability and internal consistency. These scores ranged from 0.71 (social function subscale) to 0.89 (physical function subscale) in a population of people with psychosis (Hewitt, 2007). Although a lot of the validity and reliability research has been carried out on the SF-36 rather than the SF-12, Ware et al (1996) found the MH and PH of the SF-12 were highly correlated with their respective components on the SF-36. In addition, Salyers et al (2000) found that test-retest reliability was 0.79 for both PH and MH in people with psychosis on the SF-12.

3. QoL is a subjective phenomenon and any approach has to include patients self-reports. The SF-12 is a subjective measure, an objective measure was not appropriate for this study.

4. The scale has to be adapted to the life of psychotic patients - some can be taxing on the compromised cognitive ability of many patients with psychosis.

The SF-12 only entails 12 items. Therefore it was beneficial for people with psychosis who are known to have a limited attention span (Mialet et al, 1996;

Rund et al, 1998). In addition, the most frequently used measures in the research discussed in chapter two are the SF-36 or the SF-12.

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Before the study took place, all of the measures used were piloted amongst a group of SUs to assess comprehension and the length of time to complete the measures. On average the questionnaire pack took 20 minutes to complete, with the SF-12 taking under 5 minutes. The participants reported that they found it clear and easy to understand.

5. Scale to be consistent with theoretical framework that the researcher uses to understand and define QoL in the study. The summary domains and the subscales were consistent with the theoretical framework of recovery, and therefore are consistent with the fifth recommendation. In addition, the SF-12 assessed mental health, physical health and social functioning which are considered important aspects of QoL with respect to PA and psychosis as discussed in chapter two.

6. The scale has to be sensitive to pick up relatively small changes. This point was not considered upon the choice of measure as the objective was not to assess change in QoL over a period of time, but to obtain a cross-sectional assessment of QoL.

Scoring of the SF-12

Procedures for the scoring of the SF-12 were followed from the user’s manual for the SF-12 (Ware et al, 2009).

Following data input, re-coding of the reverse items was undertaken. The protocol outlined that raw scores should be calculated for each of the 8 subscales and these were transformed to a score of 0-100. These transformed scores were standardised and a norm-based score was calculated for each of the sub-scales. The advantage of using the norm-based score is that the results for one scale can be meaningfully compared with the other sub-scales and the two summary measures of PH and MH and their scores can be directly compared to the general population. A linear z-score transformation was used in order for all eight subscales to have a mean of 50 and a SD of 10, based upon the 1998 general US population. Although it could be argued norms from the UK should be used, it is strongly recommended by the authors (Ware et al, 2009) that to

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promote consistency across measures that the norms for the US are used for ease of comparison.

To compute the two summary scores of PH and MH, the z-score of each SF-12 was multiplied by its respective physical or mental factor score coefficient to gain an aggregate physical and mental health summary score. These aggregate scores were transformed to a norm-based score with a mean of 50 and a SD of 10.